Add tab support

Add tab support

frontends/LyXView.h
	- add tab update function

frontends/qt4/GuiView.h
	- add tab update function
	- add function for setting up the tabs
	- add slot for the tab clicks
	- add pimpl

frontends/qt4/GuiImplementation.C
	- don't set the buffer as central widget
	  but initilize the tabs

frontends/qt4/GuiView.C
	- use QTabBar for switching
	- scan the bufferlist on updates

frontends/LyXView.C
	- also update the tabs

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15637 a592a061-630c-0410-9148-cb99ea01b6c8