ui: Disable markup in widgets derived from AdwPreferencesRow

This is a follow-up to !1280. It bumps the libadwaita dependency to
1.2.0 so that new API can be used to disable markup in widgets derived
from `AdwPreferencesRow`, rather than having to explicitly escape all
label contents.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>