Allow changing keyring password

Closes #15 (closed) and #7 (closed)

Contrary to original assumptions, we can change the password of a keyring without having to make any changes at file level. I think this also avoids some potential issues that could have occurred as a result.

For this we make use of the so called "org.gnome.keyring.InternalUnsupportedGuiltRiddenInterface" dbus interface which gets implemented by gnome-keyring-daemon.

Seahorse and a few other GNOME modules also make use of this interface to change the passwords of collections, for example gnome-initial-setup for setting the initial login keyring password. Therefore, I think we should be on the safe side that the interface will not disappear overnight.

It also has the advantage that the native shell dialogs are getting used: