Fix clipboard/selection encoding

Fix clipboard/selection encoding

	* src/frontends/qt[34]/qt_helpers.[Ch]
	(toqstr): add variant for docstring
	(qstring_to_ucs4): Use docstring and port from qt4 to qt3

	* Many other files: Many std::string -> lyx::docstring conversions

	* src/support/lstrings.[Ch]
	(subst): Add variant for docstring and char_type
	(externalLineEnding): std::string -> lyx::docstring
	(internalLineEnding): std::string -> lyx::docstring

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14871 a592a061-630c-0410-9148-cb99ea01b6c8