Restore the persistent selection feature (disabled because of bug 3877)

Restore the persistent selection feature (disabled because of bug 3877)
See also http://www.lyx.org/trac/changeset/19003

	* BufferView.cpp (setBuffer): save selection before switching buffer
	(mouseSetCursor): save selection before resetting it
	(putSelectionAt): no need to save selection

	* Cursor.cpp (selHandle): save selection before resetting it

	* CutAndPaste.cpp (cutSelection, eraseSelection): save selection before
	deleting it.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@19026 a592a061-630c-0410-9148-cb99ea01b6c8