... use an AttachedVirtualFunction to compute the default view height from
the controls.
This frees LabelTrackControls from cycles.
Also made Track::DoSetHeight non-virtual
... non-intrusively in the Track suclasses, registering functions instead.
For each abstract factory function (of two), build a table of concrete factory
functions, paralleling the hierarchy of Track subclesses. Dispatch using
runt time type information in the Track objects.
... that is, a factory function, open, close, import, undo/redo/rollback.
Also the callbacks from AudioIO, which need to invoke undo history push when
recording stops.
It is meant as a high-level class using several of the other things attached
to the project, while AudacityProject will be a low level class acting mostly
as just the container of the attached structures.
... Because all hit tests returned all fields blank, or else, returned a
UIHandle object whose Preview method gives the rest of the information; so
the other fields were redundant.
... Let cell hit tests, and handle preview, know states only, not transitions.
Cell hit tests are passed a mouse state that does not always match the current,
but anticipates the button click to come; usually left, but if the Control
[sic] key on Mac is down, then right.
Thus, pressing and releasing Mac Control in multi-tool switches in and out of
the magnifier cursor.
...no actions reimplemented to them yet.
Later commits will move special cases one at a time from TrackPanel, preserving
all click and drag capabilities at each step. With a few exceptions, but those
lost abilities are restored in yet later commits. (Ctrl+Click on the Label
track being one.)
