timeLimitsManager: Hold inhibitor for sleep to allow saving screen time data

timeLimitsManager: Hold inhibitor for sleep to allow saving screen time data

This avoids the race between systemd emitting the `prepare-for-sleep`
signal, gnome-shell then starting to write the screen time data to disk,
and systemd suspending the hardware.

The race isn’t so much of an issue if the suspend succeeds (if
gnome-shell loses, the data will still get written out when the machine
resumes), but it’s slightly problematic if the machine loses power while
suspended, as that means the latest screen time data is lost.

Includes significant suggestions from Florian Müllner.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3643>