Query data from Briar outside of UI thread

This MR introduces some convenience functions to access the Briar core API in background threads, reusing the existing DatabaseExecutor and IoExecutor. It also makes sure the EventExecutor is bound to the UI thread to avoid concurrency problems. All the currently existing view models have been updated to make use of the new background functionality.

The changeset is quite big, so I will go on and put some comments on key parts right now.

Closes #98 (closed)