This commit is purely mechanical and get rid of lyx_gui.[Ch].

Only qt4 is guaranted to compile and work. I did not remove gtk and qt3 lyx_gui.C because they might be needed for reference to complete the header declarations in "GuiApplication.C".

 - lyx_gui::use_gui transfered to lyx::use_gui in lyx_main.C
 - all remaining lyx_gui functions transfered to Application and corresponding GuiApplication implementations.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15306 a592a061-630c-0410-9148-cb99ea01b6c8