simplify exit code, old code doesn't work on the Mac

simplify exit code, old code doesn't work on the Mac

* lyxfunc.C                         : rename unused LFUN_LYX_QUIT parameter
* frontends/qt4/GuiImplementation.C : call LFUN_LYX_QUIT when last window closed
* frontends/qt4/GuiWorkArea.C       : check theApp, add debug case
* frontends/qt4/GuiApplication.C    : remove quitLyX()
* frontends/qt4/GuiApplication.h    : remove quitLyX()
* frontends/qt4/GuiView.C           : hide() widow fore exiting to prevent crash

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16044 a592a061-630c-0410-9148-cb99ea01b6c8