Stop sending release events for mouse wheel 'buttons' in X mouse mode.

On Windows (X mouse reporting of the mouse wheel isn't currently done
by the Unix front end, though I'm shortly about to fix that too) a
mouse wheel event is translated into a virtual button, and we send
both a press and a release of that button to terminal.c, which encodes
both in X mouse reporting escape sequences and passes them on to the
server. This isn't consistent with what xterm does - scroll-wheel
events are encoded _like_ button presses, but differ semantically in
that they don't have matching releases. So we're updating to match
xterm.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10138 cda61777-01e9-0310-a592-d414129be87e
Enable xterm mouse reporting of wheel actions in GTK.

I had somehow missed this completely out of the GTK mouse-button
handling and never noticed until now!

Of course, like any other mouse action, if you want it to be handled
locally rather than passed through then you can hold down Shift.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10139 cda61777-01e9-0310-a592-d414129be87e