Cancellation of resources screen loading

[kirillt]

When a user switches to resources screen, either by clicking [navigate folder] button on individual folder or by clicking [navigate data] in menu (aggregated mode), resources indexing, previews generation and resources sorting start. The user may change her ming in this moment and attempt to switch back to folders screen.

Right now, such attempt would not succeed. The app should support this.

In case of loading cancellation, all spawned coroutines must be cancelled (but not reverted).