Extend Table/TreeTable's interface.

- Add NotifyNodesCleared() hook to table model as a shortcut for
removing all rows.

- TreeTable::_RemoveChildRows() now recognizes the special case of the
above and optimizes it by calling BColumnListView::Clear() rather than
removing each row individually.

- Add TableModelReset()/NotifyTableModelReset(). This notification is
used to tell the underlying table that a full rebuild is needed due to
the model changing completely.
Cleanups/optimizations for ImageFunctionsView.

- Get rid of the functions array as we no longer really needed it except
to sift duplicates. The latter function is now done simply by keeping a
set of already seen function addresses, and skipping entries which fall
in said category.

- Use NotifyNodesCleared()/NotifyTableModelReset() as appropriate.

- Remove now-unused sorting functions.

Combined, these changes significantly reduce the overhead of switching
the active image, which was produced observable lag when either choosing
another image in the Images list, or when stepping into/out of a
function resulted in an image change.
VariablesView: Use Notify{NodesCleared,TableModelReset}().