where finish_stroke() is the former flush_event_queue() and
start_stroke() is used in BUTTON_PRESS instead of poking into the
motion buffer's internals. Also, call finish_stroke() also when the
tool is not active, so the event buffer is flushed (the "motion"
callback will ignore the events if the tool is not active).
and emit the buffer's "motion" signal when a motion is supposed
to happen. In GimpDisplayShell, connect to GimpMotionBuffer::motion()
and call the tool.
This commit only adds the class and removes the members from
GimpDisplayShell, so everything looks more ugly than before, but
I wanted the member moving separate from any refactorings.
