feat: add actions to dismiss/cancel the add account form

Users may now cancel adding an account and dismiss the page.
This change is particularly relevant in the context of an account that is being added via URI passed on the commandline: the user may now explicitly reject it.

Additionally quitting Keysmith from the add account form is now also supported, hidden behding a boolean flag.
This will be useful for the initial page when receiving an account via URI from the commandline: the user may reject the account and quit Keysmith via a single action.

Issues: #7, #14