* SciTECO needs to avoid automatic scrolling as an optimization.
  While this works partially by avoiding certain messages like SCI_GOTOPOS (in favor of SCI_SETEMPTYSELECTION),
  there aren't alternatives for all messages that scroll.
  For instance SCI_UNDO will always call EnsureCaretVisible().
  Also, even if we could avoid all scrolling messages, there is no guarantee that will not suddenly
  and unexpectedly break in the future.
* Instead, every scrolling is now disabled except for an explicit SCI_SCROLLCARET.
  SciTECO can therefore use the more intuitive messages like SCI_GOTOPOS.
* SetLastXChosen() (choosing the X that caret sticks to) has been found to be a major slow down
  during UNDO and serves no purpose on SciTECO anyway - we have to implement such an algorithm
  at the SciTECO language level.
  We therefore simply disable SetLastXChosen().

* This sets the default key representations on every SCI_SETDOCPOINTER
  which is very inefficient considering the way that SciTECO uses the
  Q-Register view.
  Furthermore it meant, we had to reset the representations to their
  SciTECO versions again after every SCI_SETDOCPOINTER.
* This patch only does not cause problems because we initialize
  the representations anyway for every Scintilla view.
  This patch is not meant to be upstreamed!

attempted on a position outside the document.
Positions are checked earlier to prevent actions partly succeeding.
This is implemented with a new exception type Failure which should be caught by
platform layer's API handling code to produce a more granular error status.

drop disabled to deselect on mouse down.

Allows rendering tabs (ASCII 9) with character representations like any other
control character.

SCI_SETDRAGDROPENABLED. Fully implemented on Win32 but may only prevent dragging
on other platforms.