sharing: Remember the password on remote desktop sharing

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.
11 jobs for wip/carlosg/recover-remote-desktop-password in 3 minutes and 26 seconds (queued for 3 seconds)
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