Improve Inset dissolve capability (bug 2201).

Improve Inset dissolve capability (bug 2201).

	* src/insets/insettabular.C (getStatus):
	* src/insets/insettext.C (doDispatch,getStatus): remove special code
	for LFUN_INSET_DISSOLVE.

	* src/text.C (dissolveInset): new method.
	(erase, backspace): use dissolveInset.

	* src/text3.C (dispatch): use dissolveInset for LFUN_INSET_DISSOLVE.
	(getStatus): disable LFUN_INSET_DISSOLVE if the inset has more than one 	cell.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14940 a592a061-630c-0410-9148-cb99ea01b6c8