Rework the Unicode hex input code. Now we only steal a single key

Rework the Unicode hex input code. Now we only steal a single key

2005-09-02  Matthias Clasen  <mclasen@redhat.com>

	* gtk/gtkimcontextsimple.c: Rework the Unicode hex input
	code. Now we only steal a single key combination, Ctrl-Shift-U,
	instead of sixteen.
	A hex Unicode sequence must be started with Ctrl-Shift-U, followed
	by a sequence of hex digits entered with Ctrl-Shift still held.
	Releasing one of the modifiers or pressing space while the modifiers
	are still held commits the character. It is possible to erase
	digits using backspace.
	As an extension to the above, we also allow to start the sequence
	with Ctrl-Shift-U, then release the modifiers before typing any
	digits, and enter the digits without modifiers.
	(#82011, Owen Taylor)