Add time & date indicator toggles

This requires ubports/indicator-datetime#37 and ubports/indicator-datetime#40 (old Indicators)

I am currently working on a forward port of those PRs for ayatana-indicator-datetime, as well, if needed.

Luigi311:

This is a forward port of !371 (closed) which is a forward port of !288 (closed)

Includes change requested in 371

Forwarded the required patches to ayatana-indicator-datetime
https://github.com/AyatanaIndicators/ayatana-indicator-datetime/pull/117 (merged)
https://github.com/AyatanaIndicators/ayatana-indicator-datetime/pull/118 (merged)