Cancellation of resources screen loading

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).

Trace the resources screen loading flow for indexing, preview generation, and sorting, then identify the spawned coroutines and the transition back to the folders screen. Done means switching back succeeds and all loading coroutines are cancelled without reverting their completed work.

Written by the indexing model from the issue text.