* frontends/FontLoader.h: new interface class.
* frontends/Application.h: new FontLoader() pure virtual method
* frontends/lyx_gui: update_fonts() and font_available() methods deleted
* [qt3/qt4/gtk]/GuiApplication: implement the new interface
* qt4/GuiFontLoader:
- renamed from FontLoader
- now derives from FontLoader
- now in the lyx::frontend namespace
* qt3/qfont_loader
- now derives from FontLoader
* gtk/xftFontLoader
- now derives from FontLoader
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15220 a592a061-630c-0410-9148-cb99ea01b6c8