schemas: Add break-reminder schema
This will allow implementing break reminder functionality in the shell, while having the settings for it in gnome-control-center.
The preferences layout is strongly inspired by what’s used by
org.gnome.BreakTimer
(https://gitlab.gnome.org/GNOME/gnome-break-timer/).
See gnome-shell!3251
Signed-off-by: Philip Withnall pwithnall@gnome.org
Helps: Teams/Design/initiatives#130