When using freehand drawing of automation, the line itself
is not notified (no ::start_drag(), ::end_drag() calls).
This adds a end_draw() call which allows the AutomatioLine
to perform additional actions.
This involved a significant change in event handling for automation region views,
but it brings it into line with how it works for other things. On button press
we initiate a drag, then if no motion occurs, the Drag returns false during
finalization, and only then do we continue through Editor::button_release_handler()
to eventually end up in ::add_automation_event().
Although it is a substantial change, the fact that it now works the same
way for audio regions, automation regions and automation tracks seems
like a definite plus.
