Allow highlighting calendar cells

Provides an API for highlighting calendar cells in the grid, e.g. to mark appointments.

This is more of a draft, obviously docs, tests etc. are missing. I was just wondering if this is something that you would accept in general.

Notifying the sub-views of a changed highlight state is currently done by `notifyDataSetChanged`ing and re-setting the `MonthAdapter`. If anyone has an idea about how to do this more elegantly, it'd be very welcome.