Improve month view and schedule view visually

This MR mainly contains visual tweaks to the Month View, as well as a bug fix affecting the length of multi-day event lines. Right clicking on an empty space in the month view grid also now presents the incidence creation actions.

New monthview:

Old monthview:

There are also minor changes to the schedule view.

New scheduleview (highlighted today):

Finally, both views now have a 'Today' main action that resets the view to the current day.