Fix crash on exit (bug 2549) by correct usage of QApplication

Fix crash on exit (bug 2549) by correct usage of QApplication
	* src/lyx_cb.C
	(quitLyX): lyx_gui::exit takes now an argument

	* src/frontends/{gtk,xforms}/lyx_gui.C
	(lyx_gui::parse_init): rename to lyx_gui::exec and call LyX::exec2
	(void lyx_gui::exit): add exit status argument

	* src/frontends/qt{3,4}/lyx_gui.C
	(cleanup): new function for pointer cleanup
	(lyx_gui::parse_init): rename to lyx_gui::exec and call LyX::exec2,
	turn static variables into automatic variables
	(void lyx_gui::exit): add exit status argument
	(start): Use cleanup()
	(exit): ditto

	* src/frontends/lyx_gui.h
	(parse_init): remove
	(exec): new
	(exit): Take exist status argument

	* src/lyx_main.[Ch]
	(LyX::priv_exec): split into LyX::priv_exec and LyX::exec2

	* src/lyx_main.C
	(lyx_exit): New, choose the right exit function
	(showFileError): call lyx_exit
	(LyX::queryUserLyXDir): ditto
	(LyX::init): ditto
	(LyX::priv_exec): ditto
	(LyX::priv_exec): Replace want_gui by lyx_gui::use_gui
	(LyX::priv_exec): replace lyx_gui::parse_init by lyx_gui::exec and
	exec2
	(LyX::init): Replace gui argument by lyx_gui::use_gui

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14036 a592a061-630c-0410-9148-cb99ea01b6c8