Add local read-only calendar

[PanderMusubi]

Add local read-only calendar in a similar way that https://f-droid.org/repository/browse/?fdid=org.birthdayadapter does. Calender events that should be offered are three multi-day all-day events (red, green and blue). In preferences could be chosen by the user to show red, red+green, red+blue or red+green+blue, the last one is the default).

Only for the current and maximum the x past/last three cycli should be offered. This could also be configurable by the user. For example the number of cycli to offer in the calendar. 0 is none, 1 is only the current, 2 is the current plus previous, 3 is the current plus previous two (default) et cetera with a maximum of 12?

On each multi-day all-day event has only minimal information, preferably only a color and one word that is not obvious for other accidental viewers/glancers of the calendar view. A link (url) could be added in each event that opens the local app.

Implementing this direct view in calendar makes it easier to see for the users when what is to be expected or was in the past. Lots of code can be reused from Birthday Adapter.

[arnowelzel]

Sorry for the long delay - I was quite busy with many other things in the past. I'll try to add this for the next update.