Add Plasma Calendar plugin Astronomical Events

Summary:
For now adds the option to show
* lunar phase events (full moon, new moon, quartermoon)
* astronomical season (solstices, equinoxes)

Future plan: also show sunset & sunrise for given location

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12433