Trigger manual data refresh with Ctrl+r

## Problem

It would be great to allow the user to manually refresh the data while the app is running.

## Proposal

This can be done with an accelerator action for `Ctrl+r`. We might also want to indicate refresh activity by showing a spinner animation (can be the same as in #171).

## Prior Art

Backend refresh triggers are already used after renaming a Task List. Maybe we can just use the same technique:

https://github.com/elementary/tasks/blob/0a11aefa6d497f02555d674463bd90bbd1587cdd/src/TaskModel.vala#L180

Also some accelerators (keyboard shortcuts) have already been implemented in `Tasks.MainWindow`, so we can use those to figure out how to create another one:

https://github.com/elementary/tasks/blob/0a11aefa6d497f02555d674463bd90bbd1587cdd/src/MainWindow.vala#L23