Add Week Widget

[jwsp1]

Etar needs some more widgets rather than just a plain event list.
Coming from aCalendar, I'd really like to see a widget containing an overview of the current week's events.
I know there is #162 but this only covers a month view and it has been around here for 4,5 years. Also, #428 doesn't seem to be "in progress" (2,5 years old).