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