This commit introduces the FontLoader interface class. In the future, I intend to...

This commit introduces the FontLoader interface class. In the future, I intend to extend it with a proper encapsulation of font metrics.

* 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