Make string conversion work with non-ucs2-characters if using qt 4.2

Make string conversion work with non-ucs2-characters if using qt 4.2

	* src/frontends/qt4/qt_helpers.[Ch]
	(toqstr): Use QString::fromUcs4 if the qt version is at
	least 4.2.
	(qstring_to_ucs4): Use QString::toUcs4 if the qt version is at
	least 4.2
	(ucs4_to_qstring): Delete to avoid confusion, since it was only
	used in one place

	* src/frontends/qt4/panelstack.C
	(PanelStack::addCategory): Use toqstr instead of ucs4_to_qstring

	* src/support/unicode.[Ch]
	(ucs2_to_ucs4): Replace with utf16_to_ucs4
	(ucs4_to_ucs2): Replace with ucs4_to_utf16

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16169 a592a061-630c-0410-9148-cb99ea01b6c8