Switch to reactive framework for UI and other updates

[mpcjanssen]

The timing and triggering of UI updates and todo updates based on events and modifications is becoming fairly convoluted leading to several issues (e.g. #820, #901).

The proper way to fix this is to make all updates based on reactive streams.

• [x] Update drawers based on caused by changes to the todolist.
• [x] UI updates based on todo list changes
• [ ] Delayed file safe to reduce network traffic.

[mpcjanssen]

Seems that removing some layers of events might be an equally valid approach.

[smichel17]

Which layers?

[mpcjanssen]

the use of broadcast events just to update the local UI and the at this point overuse of invalidateOptionsMenu. Just updating the appropriate parts of the UI seems more direct and easier to reason about.

[mpcjanssen]

Using a single point to trigger UI updates using a certain type of event seems to work very well without adding further indirection