This commit adds a function that resets key bindings to the default in
`test_keybinding_edit_produce_expected_accels` test.
Note that this function needs to run at the end of every test that
modifies the key bindings as they are not automatically reset after
the test has run.
This commit adds
`test_keybinding_successfully_reassigned_after_clearing` test,
which checks that a key binding is successfully reassigned after it
has been cleared.
This commit adds tests that check that:
1) Default key binding accelerators are distinct.
2) Message dialog appears when a duplicate key binding accelerator is
attempted to be assigned to a different action.
3) Duplicate key binding accelerator cannot refer to more than one action.
Also, `active_message_dialog` attribute was added to `PrefsEditor`, so
that tests could get a reference to a message dialog window.
