Reduce calls of ::exit()

Reduce calls of ::exit()
	* src/main.C
	(main): return LyX::exec()

	* src/frontends/{gtk,qt3,qt4}/lyx_gui.C
	(lyx_gui::exec): Change return value to int
	(lyx_gui::start): ditto

	* src/frontends/qt[3,4]/lyx_gui.C
	(lyx_gui::exit): Call QApplication::exit instead of ::exit, this
	hack is not needed anymore

	* src/frontends/lyx_gui.h
	(lyx_gui::exec): Change return value to int
	(lyx_gui::start): ditto

	* src/lyxtextclasslist.[Ch]
	(LyXSetStyle): SChange return value to bool and don't call ::exit

	* src/lyx_main.C
	(showFileError): Don't exit anymore

	* src/lyx_main.[Ch]
	(LyX::exec): Change return value to int
	(LyX::priv_exec): ditto
	(LyX::exec2): ditto
	(LyX::init): Change return value to bool
	(LyX::readRcFile): ditto
	(LyX::readUIFile): ditto

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14339 a592a061-630c-0410-9148-cb99ea01b6c8