Persistent-selection (and fix bug 3162)

Persistent-selection (and fix bug 3162)
	* src/CutAndPaste.h/C: add selectionBuffer to save selected text
	* src/text3.C: proper handling of paste
	* src/lyxfind.C: save selection
	* src/BufferView.C: save selection
	* src/text.C: save selection
	* src/cursor.C: save selection
	* src/insets/insettabular.C: save selection
	* src/mathed/InsetMathGrid.C: save selection
	* src/mathed/InsetMathHull.C: save selection
	* src/mathed/InsetMathNest.C: save selection

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17022 a592a061-630c-0410-9148-cb99ea01b6c8