Factorize general code out of GuiDialog and into the base Dialog class. This is now...

Factorize general code out of GuiDialog and into the base Dialog class. This is now possible thanks to the moving of Dialog.{cpp,h} to qt4/. Any dialog must inherit QWidget within Qt so it is safe to use Dialog::asQWidget() method (which is similar to dynamic casting).

* GuiDialog now inherits DialogView. GuiDialog is only there to bring the ButtonController archaeological interface; any new dialog should inherit DialogView and any new dock widget should inherit DockView.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@22049 a592a061-630c-0410-9148-cb99ea01b6c8