* As known from DEC TECO, but extended to convert absolute positions to line numbers as well.
  :^Q returns the current line.
* Especially useful in macros that accept line arguments,
  as it is much shorter than something like
  ^E@ES/LINEFROMPOSITION//+Q.l@ES/POSITIONFROMLINE//:^E-.
* On the other hand, the fact that ^Q checks the line range means we cannot
  easily replace lexer.checkheader with something like
  [:J 0,^Q::S...$ ]:
  Using SCI_POSITIONFROMLINE still has the advantage that it returns `Z` for out-of-bounds ranges
  which would be cumbersome to write with the current ^Q.
* Perhaps there should be a separate command for converting between absolute lines and positions
  and :^Q should be repurposed to return a failure boolean for out-of-range values?
* fnkeys.tes could be simplified.

* The colon modifier can now occur 2 times.
  Specifying `@` more than once or `:` more than twice is an error now.
* Commands do not check for excess colon modifiers - almost every command would have
  to check it. Instead, a double colon will simply behave like a single colon on most
  commands.
* All search commands inherit the anchored semantics, but it's not very useful in some combinations
  like -::S, ::N or ::FK.
  That's why the `::` variants are not documented everywhere.
* The lexer.checkheader macro could be simplified and should also be faster now,
  speeding up startup.
  Eventually this macro can be made superfluous, e.g. by using 1:FB or 0,1^Q::S.

* lexer.checkheader is therefore case-sensitive now as well

* We cannot call it "." since that introduces a local register
  and we don't want to add an unnecessary syntactic exception.
* Allows the idiom [: ... ]: to temporarily move around.
  Also, you can now write ^E\: without having to store dot in a register first.
* In the future we might add an ^E register as well for byte offsets.
  However, there are much fewer useful applications.
* Of course, you can now also write nU: instead of nJ, Q: instead of "." and
  n%: instead of "nC.". However it's all not really useful.

* You can now set a per-file tab style, that differs from the defaults established
  in the ED hook.
  This is important especially since we do not yet support per-project .teco_ini
  scripts where you could establish differing policies depending on the VCS repository.
  (The latter would be easy to implement, but we cannot currently easily extend the
  existing ED hooks.)
* It's unlikely that files contain an ASCII 27, but not impossible.
  Therefore we now use ASCII 0 (^@) as a terminator.
  This indeed be safe under UNIX.
  Even better would be a string building construct for escaping ASCII 27 ($), though,
  as that would work with arbitrary bytes.

The macro prefix was changed from ^F to ^K.

* Curses: "icons" have also been added

This previously highlighted little more than embedded Javascripts.

* ALL keypresses (the UTF-8 sequences resulting from key presses) can now be remapped.
* This is especially useful with Unicode support, as you might want to alias
  international characters to their corresponding latin form in the start state,
  so you don't have to change keyboard layouts so often.
  This is done automatically in Gtk, where we have hardware key press information,
  but has to be done with key macros in Curses.
  There is a new key mask 4 (bit 3) for that purpose now.
* Also, you might want to define non-ANSI letters to perform special functions in
  the start state where it won't be accepted by the parser anyway.
  Suppose you have a macro M→, you could define
  @^U[^K→]{m→} 1^_U[^K→]
  This effectively "extends" the parser and allow you to call macro "→" by a single
  key press. See also #5.
* The register prefix has been changed from ^F (for function) to ^K (for key).
  This is the only thing you have to change in order to migrate existing
  function key macros.
* Key macros are enabled by default. There is no longer any way to disable
  function key handling in curses, as I never found any reason or need to disable it.
  Theoretically, the default ESCDELAY could turn out to be too small and function
  keys don't get through. I doubt that's possible unless on extremely slow serial lines.
  Even then, you'd have to increase ESCDELAY and instead of disabling function keys
  simply define an escape surrogate.
* The ED flag has been removed and its place is reserved for a future mouse support flag
  (which does make sense to disable in curses sometimes).
  fnkeys.tes is consequently also enabled by default in sample.teco_ini.
* Key macros are handled as an unit. If one character results in an error,
  the entire string is rubbed out.
  This fixes the "CLOSE" key on Gtk.
  It also makes sure that the original error message is preserved and not overwritten
  by some subsequent syntax error.
  It was never useful that we kept inserting characters after the first error.