Remove what only the Qt frontend used

- The theme/font request globals (gThemeChange*, gFont*Request,
  gFontChangePending, gCurrentFont*, gFontDialogRequested): only Qt
  read them.
- The font-picker-toggle command: it only set gFontDialogRequested, so
  it did nothing in kge or kte.
- The terminal :font and :font-size recorded requests nothing read and
  answered "Font requested"; they now say fonts are not available in
  the terminal.
- kte::DisplayColumns, added for Qt's drawing and now unused.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Command.h

@@ -28,7 +28,6 @@ enum class CommandId {
 	OpenFileStart, // begin open-file prompt
 	VisualFilePickerToggle,
 	// GUI-only: toggle/show a visual font selector dialog
-	VisualFontPickerToggle,
 	// Buffers
 	BufferSwitchStart, // begin buffer switch prompt
 	BufferNew, // create a new empty, unnamed buffer (C-k i)