Implement LFUN_INSET_DISSOLVE (bug 2201):

Implement LFUN_INSET_DISSOLVE (bug 2201):
        * src/LyXAction.C:
        * src/lfuns.h:
		- add new lfun LFUN_INSET_DISSOLVE.

        * src/insets/insettext.C
        (void InsetText::doDispatch):
		- dissolve inset when hitting backspace in the
		  very first or delete in the very last position
		  of an inset.

        * src/text3.C
        (void LyXText::dispatch):
        (bool LyXText::getStatus):
		- implement new lfun LFUN_INSET_DISSOLVE.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14572 a592a061-630c-0410-9148-cb99ea01b6c8