[gpm][DIFF] paste olny if ctrl is pressed

[Alessandro Rubini]

> i have a new feature to gpm, to Paste only when CRTL is pressed, to
> avoid unwanted pastes...

No need to patch gpm. Just register a "default handler" that uses
mouse-only events and doesn't do anything with them. That way, only
mouse-and-modifier get get to the selection engine.

Please check the documentation about how to register client, and look
at included applications (mev and gpm-root) to look at real
implementations.

/alessandro