Add support for Thunderbird's Lightning calendar to indicator-datetime

[Alexander Kallenbach]

Thunderbird is the default E-Mail-Client, as was Evolution before. In previous Ubuntu-Versions you could click on the indicator and could see your tasks and dates and it was even possible to edit your dates.
Nothing of this is possible now with the default installed apps.
Big regression.

<https://wiki.ubuntu.com/TimeAndDate#settings>: "The “Coming events from” menu should default to “Evolution” if it is installed, otherwise “Lightning” if it is installed, otherwise “Subscribed Calendars”."

<https://wiki.ubuntu.com/TimeAndDate#Coming_events>: "Whenever “Coming events” is checked and the selected calendar source has events, the menu should show the next five calendar events, plus the “Add Event…” item if the calendar source is an application..."

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[John Lea (johnlea)]

Hi MPT, could you take a look at this bug, thx!

[Matthew Paul Thomas (mpt)]

Specification updated. <https://wiki.ubuntu.com/TimeAndDate?action=diff&rev2=45&rev1=44>