Original problem: widgets do not get synthesized mouse-down and
mouse-press events until the stylus is released

Reason: if the app accepts the event, WndProc should report
that to the system (by returning true). This is the only way to
prevent Windows from starting some system-wide gestures, like
click+hold -> right button click. If we ignore the event, then
OS postpones all synthesized mouse events until the entire gesture
is completed.

The patch implements a "hackish" workaround for the original problem
by using the following rules:

1) All tablet-move events are ignored (without synthesized mouse events
   OS doesn't generate any Enter/Leave events)

2) All not-accepted tablet press- and release-events and also reported as
   ignored (without it D&D doesn't work).

3) All accepted tablet press- and release-events are reported as "accepted",
   **but** we artificially synthesize mouse events for them.

TODO: there are still two problems:...