* Also updated A_XXX attributes to WA_XXX, where the "new" wattr_set() APIs
  are used. This doesn't make a difference on ncurses and PDCurses, but the
  X/Open standard demands it.
* Allow color pairs up to 32767 instead of only up to 32766.
* We now always require Curses wide-character APIs due to Scinterm,
  which has to use mvwin_wch(). We were practically depending on wide-character
  support anyway, so this shouldn't really restrict portability.
* teco_curses_add_wc() has been simplified, now that we can rely on
  wide-char APIs. Perhaps it should be removed altogether?

* When constructing the list of popup items, the unnamed buffer is stored as the empty string
  instead of a prerendered "(Unnamed)".
  Using the empty string simplifies autocompletions, which will actually have to insert nothing
  at all (in addition to terminating the string).
* Since unnamed buffers are now special in the popup list, we can render them with special
  icons as well.
  Currently, only on Curses we use a file symbol with a question mark.
  There doesn't appear to be a fitting standard Freedesktop icon to use on GTK and there
  isn't even any fitting standard emblem to lay over the default file icon.

* Unfortunately we cannot use `wbkgdset(win, A_REVERSE)` if we plan to
  use reverse text on this background, i.e. if we want to cancel out the background A_REVERSE.
* SciTECO therefore no longer uses background attributes, but only foreground attributes.
  When setting a reverse text, we XOR A_REVERSE into the previous attributes.
* This fixes control characters especially in the info line and popups, as well as rendering
  of the popup scroll bars.
* The command-line should now be rendered properly even on a dark-on-bright color theme
  (which does not yet exist).

* Turns out that "%C" in wprintw() does not work with non-ANSI chars.
* We still don't want to introduce the Curses widechar API,
  so I added teco_curses_add_wc() as a replacement for wadd_wch().

* All non-ASCII characters are inserted as Unicode.
  On Curses, this also requires a properly set up locale.
* We still do not need any widechar Curses, as waddch() handles
  multibyte characters on ncurses.
  We will see whether there is any Curses variant that strictly requires
  wadd_wch().
  If this will be an exception, we might keep both widechar and non-widechar
  support.
* By convention gsize is used exclusively for byte sizes.
  Character offsets or lengths use int or long.