If we are going through mutter's RemoteDesktop interface, we don't
seemingly remember the password set. Add support for reading it
from secrets and change the entry password on dialog construction,
to bring it on par with our vino handling.
Enable support for manipulating GNOME Remote Desktop settings. Settings
are done via the org.gnome.desktop.remote-desktop.vnc schema.
Configuring the VNC password is done via libsecret, thus libsecret is
added as a dependency.