Coroutines finalizer

[kirillt]

In case of the app crashing, we can lose coroutines which haven't been completed. We might need to write more asynchronous code but we can't allow modules like tags storage to lose new data.

We should implement global exception handler ("last frontier"), which knows about spawned coroutines and ensures they complete before crashing. The same handler would send crash reports (#78).

[kirillt]

Also this finalizer should be called when a user wants to kill the app, but we have some active coroutines. In this case, the user should see progress spinner with label "Saving".