Fix incidentals to Bug 1331, though not the bug itself...

... See comments #1 and #2 at
http://bugzilla.audacityteam.org/show_bug.cgi?id=1331

Don't make strange undo history if, e.g., R to record (or other keystroke
with undoable effects) interrupts a drag with undoable effects (like time
shift).  Ensure that by first simulating a mouse button up event to stop the
drag, before dispatching the keystroke.

Don't crash if certain other drags, that do not have undoable effects, such
as selection or vertical ruler drag -- are interrupted by a keystroke
command (Ctrl+C in particular could cause crash).  However, in these cases,
the drag is still allowed to continue.

src/Track.cpp

@@ -1167,7 +1167,7 @@ bool TrackList::MoveDown(Track * t)
    return false;
 }
 
-bool TrackList::Contains(Track * t) const
+bool TrackList::Contains(const Track * t) const
 {
    return std::find_if(begin(), end(),
       [=](const value_type &track) { return t == track.get(); }