The uncontroversal part of the selection fix: Always set the X selection

The uncontroversal part of the selection fix: Always set the X selection
after selecting something with the mouse

	* src/insets/insettabular.C
	(InsetTabular::doDispatch): Set the X selection if there is a
	selection after release of the left mouse button

	* src/mathed/InsetMathNest.C
	(InsetMathNest::doDispatch): Set the X selection after double and
	triple click of the left mouse button
	(InsetMathNest::lfunMouseRelease): Set the X selection if there is a
	selection after release of the left mouse button

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16530 a592a061-630c-0410-9148-cb99ea01b6c8