Calendar Widget: set the locale appropriate first day of the week

Also adjust the minimum size, given I changed this in commit
26855234acef ("Make date widget easier to read").

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Calendar Widget: get rid of week number

It puzzles me that this is the default.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Calendar Widget: make it modal

Also fix the setWindowFlags calls - those are not incremental, so you need
to call the method once and combine the flags.

I would also like ESC to close the calendar widget, but haven't figured
out how to do that...

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>