This commit introduces frontends/Application.[Ch] and makes qt4/GuiApplication (renam...

This commit introduces frontends/Application.[Ch] and makes qt4/GuiApplication (renamed from qt4/Application) derive from it. Most of the code in qt4/lyx_gui.C has been transferred either to Application or to qt4/GuiApplication. Application handles unique instances of LyXFunc, LyXServer and LyXServerSocket.

Most of qt3 and gtk should stay compilable except for LyXView.h because the LyXFunc instance has been transferred to Application.

* frontends/Application: new class aimed to be the one unique interface between the frontend and the kernel. Contains one global pointer to the unique instanciation theApp.

* frontends/qt4/GuiApplication: renamed from qt4/Application, the qt4 specialisation of the Application class.  Contains one global pointer to the unique instanciation guiApp (equal to theApp but pointing to a GuiApplication instead).

* frontends/qt4/lyx_gui.C: most of the code has been moved to Application and GuiApplication

All other file: adapted to new API.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15114 a592a061-630c-0410-9148-cb99ea01b6c8