Break LabelsMenu into classes for open, save and manage

LabelsMenu was getting unwieldy with many conditionals for the open,
save and manage actions. LabelsMenu is now used for manage. The open
and save actions have their own LabelsMenu classes.

Adding/deleting labels has been removed from open and save menus.

This also fixes a bug with the open droplist not repopulating when
labels are unselected.
Forgot to remove alert message
Prevent users from saving models they don't own

Give them the option of using "Save as…" to save a copy of the model as
their own.