Combine small mouse wheel events

Combine small mouse wheel events

The VNC protocol can't handle different deltas or speeds for a mouse
wheel event. When using a device that sends a lot of small mouse wheel
events, instead of fewer larger steps, the effect was that mouse wheel
scrolling was way to sensitive.
This patch looks at the delta of wheel events and doesn't send events
until the combined delta has passed a threshold. Single events that
doesn't pass the threshold get sent after a timeout in order to not
loose any events.

Fixes #577.