Add event editing and deleting capabilities, major bugfixes

This MR contains a number of changes:

• Kalendar can now edit and delete existing events
• Events from read-only collections can only be viewed and not edited/deleted
• The event editor now supports displaying and adding event recurrence exceptions
• Clicking on events now provides an informative card similar (but not yet equal to) KOrganizer's
• Right-clicking on events not presents context menu allowing users to edit or delete event
• Monthview now changes months properly without altering order of weekdays
• Many, many bugs squashed

To-do:

• Move event editor sheet to separate window on desktop (DONE)
• Break up event editor components into separate QML components, same with month view event cards
• Provide user with confirmation dialog before event deletion (DONE)
• Display journals and to-dos in kalendar (Next MR?)