Win32: Add support to convert mouse messages (WM_) to ui::Messages (using she::Events)

On Windows, instead of polling Allegro mouse position/buttons we can use
the mouse messages (WM_*) directly to generate she::Events. Those events
are received by the ui::Manager and converted to ui::Messages.
Maybe this is a possible fix for issue #133 (Wacom tablets don't work
properly).

Changes:
- Don't use jmouse_z() directly (new ui::MouseMessage::wheelDelta() member)
- Add ui::_internal_set_mouse_position() to change the jmouse_x/y(0)
  from the new mouse position received in she::Events. The same for
  ui::_internal_set_mouse_buttons().
- Modify the ui::Manager to generate mouse events in any case: using old
  Allegro 4 polling method, or from she::Events.