[notifications] Add x-nemo-display-on hint

When this hint is set for a notification, the display will be turned on
when feedback is played.
[lipstick] Move notifications db to centralised storage location

Previously, the notifications db was stored under
~/.config/lipstick/notifications.db

This commit changes the location to being:
~/.local/share/system/privileged/Notifications/notifications.db
Merge pull request #97 from special/master

[notifications] Add x-nemo-display-on hint
Merge pull request #103 from chriadam/master

[lipstick] Move notifications db to centralised storage location
Merge with master