Fix bug 2884 and 3437:

Fix bug 2884 and 3437:

* src/insets/insetbase.h:
* src/insets/insetcollapsable.h:
	- make isFixedWidth (former insetcollapsable member) a member of insetbase.
src/insets/insettext.h:
	- make border_ public (needed by insetcollapsable).
src/insets/insettext.C (draw, drawSelection):
	- adjust drawing for fixed width insets
* src/insets/insetcollapsable.C (metrics):
	- adjust dimension for fixed width insets
src/insets/insetbox.C (metrics):
	- properly calculate metrics for fixed width insets.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17879 a592a061-630c-0410-9148-cb99ea01b6c8