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.
