datetime: Rename settings to clock_settings

https://bugzilla.gnome.org/show_bug.cgi?id=707252
datetime: Exit early if the timedate1 service isn't available

We cannot meaningfully change (or display) any Date & Time system
settings without the timedate1 DBus service, so just exit early.

https://bugzilla.gnome.org/show_bug.cgi?id=707252
datetime: Move polkit initialization earlier

... so that in the following commit, we can rely on the permissions
being available.

While at this, also create a DATETIME_PERMISSION for the PolicyKit
action ID at the top of the file.

https://bugzilla.gnome.org/show_bug.cgi?id=707252
datetime: Use GObject property binding for the NTP switch

This makes it possible to remove the code for manually keeping the NTP
switch and the timedated1 DBus proxy properties in sync, and also makes
it easy to share code with the automatic timezone switch.

https://bugzilla.gnome.org/show_bug.cgi?id=707252
datetime: Show the automatic timezone switch

The backend in gnome-settings-daemon is now functional, so we can expose
the option in the UI.

https://bugzilla.gnome.org/show_bug.cgi?id=707252
datetime: Use new g_date_time %z format modifier

Use new %:::z format string to display timezone offset in a slightly
nicer way. This is also needed for consistency with the timezone
notifications from gnome-settings-daemon.

Also bump glib dependency to 2.37.7 for the new modifier.

https://bugzilla.gnome.org/show_bug.cgi?id=707253
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