... Let the window respond to an undo manager event instead, whenever there
is a push or modify
Maybe this makes a few unnecessary redraws that did not happen before. If
that is important, then we should figure out how to put the logic for eliding
the redraw into ProjectWindow, and the extra information needed for the
decision into the events, but not make intrusions in other code all over the
place.
... Problem involves sequence in which event handlers are done, and started at
commit 278509a which updated track Y coordinates in a handler.
Solution is to delay the updating of scrollbars further, using CallAfter.
Also remove one unnecessary call to ProjectWindow::FixScrollbars() because
ProjectWindow::HandleResize follows it and includes scrollbar update already.
