Make 'active' property set public

Remove the deactivate() method, just handle the attribute active.
README: who to run in debug mode
controller: add on_registrars_dialog() callback
dialogs: set registrar dialog title
controller: registrars dialog is transient

to main view
dialogs: add activate_registrar() signal
model: add set_activate_registrar() method
controller: handle activate_registrar() signal

(de)activating the registrar