[Gnuplot-info] Better handling for mouse events in UI frontends

[Nikita Z. <coo...@ma...>]

For now UI's like gtk and qt try to handle all events. This may be
doable with mouse, but very hard with input devices with extreme events
rate like touchpad (one time I tested it - it seemed to reach 1000
events per second). If UI can't do it in time - events accumulate and
we get UI hung while trying to handle all accumulated movements.

Simple panning may be not so problem, because it takes absolute
event coordinates rather than displacements. As for others - I see 2
variants:
- either separate drawing from event handling
- or skip events, whose handler is busy right now