refactor: eliminate the `cola.stash` package

Move the widgets to cola.widgets.stash and the model to
cola.models.stash.

Signed-off-by: David Aguilar <davvid@gmail.com>
git-cola: rename entry point functions to avoid name clashes

This eliminates the use of "import as".

Signed-off-by: David Aguilar <davvid@gmail.com>