Use real calendar colors with adaptive text color (dark/light) (#1091)

Use real calendar colors with adaptive text color (dark/light) depending on brightness of the event background.
A setting allows to toggle between "real event colors" and the classic way of adding a dark overlay with white text.

Authored-by: sofajazz <thomas.montibeller@headstore.com>
Change-Id: I31e763058015f7f8a7dce7cf1080ade0d7e2fbc5