Fix and document middle mouse button paste. This is probably the last of

Fix and document middle mouse button paste. This is probably the last of
the status quo fixes, I'll send a proposal how to proceed soon.

	* src/CutAndPaste.[Ch]
	(copySelection): Split into copySelection and copySelectionToStack

	* src/text3.C
	(LyXText::dispatch): Use copySelectionToStack instead of LFUN_COPY
	to copy the selection to the cut buffer for two reasons:
	- LFUN_COPY did not work (probably because bv.cursor() was not yet set)
	- If it would work it would put the selection to the system clipboard
	  which is clearly wrong.
	Document why we put the selection to the stack.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16534 a592a061-630c-0410-9148-cb99ea01b6c8