Introduce Calendar settings

The calendar functionality is still sorely lacking. The list above is a good start. What on earth is the point of the current settings dialog? It's a list of calendars, each with a colour (forgive me if I am not amazed by that) and an on-off switch (what does that even do? not clear from the context).

The N9 was so good at calendar stuff that it actually changed my life (in a small prosaic way): I don't recall missing a single meeting or appointment in the 2.5 years I spent with the N9 as my primary device. Sadly it's not so with the Jolla: I've already missed one engagement, although that's due to a bug ( https://together.jolla.com/question/39236/bug-google-calendar-events-off-by-12-hours/ ) rather than the currently more-or-less pointless settings dialog.

I think week start day should rather be a global locale setting if added. Consistent then on all the date pickers.

For rest, working days wouldn't currently be used anywhere, I think, and default calendar avoids sepate setting by updating the default to notebook where last new event was created to.