Introduce a new mouse handling option, in which the right button

Introduce a new mouse handling option, in which the right button
brings up the context menu (and you can then paste by selecting
`Paste'). Should be more friendly to Windows-oriented users as
opposed to expatriate X users; also has the effect of making it more
difficult to paste into PuTTY by a single misplaced mouse click,
which has been a common theme of complaint recently.
For the moment, `Compromise' (the X-like behaviour with the right
and middle buttons reversed so that two-button users still get the
two most important functions) is still the default. I'm uncertain
that it might not be better to make the new option the default,
though, since the compromise option is optimal for _nobody_.

[originally from svn r3565]