Fix bug 4166: Crash when middle button click on inset label

Fix bug 4166: Crash when middle button click on inset label
http://bugzilla.lyx.org/show_bug.cgi?id=4662

	* InsetCollapsable.cpp (doDispatch): cleanup the handling of mouse-
	related lfuns. The new behavior is as follows:
	- everything that does not touch the button is sent to the insettext,
	if it is visible, and delegated to enclosing inset instead
	- mouse1-press is delegated to enclosing inset (which will set the
	cursor)
	- the rest is either acted upon or results on a mere cur.noUpdate().

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@26122 a592a061-630c-0410-9148-cb99ea01b6c8