We use char_type, not size_t for storing UCS4 characters.

We use char_type, not size_t for storing UCS4 characters.
This may make a difference when char_type is signed.

	* src/frontends/LyXKeySym.h
	(getUCSEncoded): change return type from size_t to char_type

	* src/frontends/qt4/QLyXKeySym.[Ch]
	(getUCSEncoded): ditto

	* src/lyxfunc.C
	(LyXFunc::processKeySym): adjust to the above and add a FIXME since
	this is strange code that looks like it was not intended to do what
	it does.

	* src/lyxfunc.h
	(encoded_last_key): Correct documentation

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16833 a592a061-630c-0410-9148-cb99ea01b6c8