Boosting priority of folders in indexing queue, basing on type and access frequency

When a user opens the app, she sees folders screen. Indexing of resources should start immediately in background. All "favorites" should be placed first in the queue, "roots" follow after. Every folder must have score which is incremented every time the user opens the folder. If types of 2 folders are the same, their scores are used to order them in the queue.

Start by tracing the folder screen and the background indexing flow, then inspect how favorites, roots, folder types, and folder opens are represented. Done means indexing begins when the app opens, favorites precede roots, and same-type folders are ordered by their access-frequency scores.

Written by the indexing model from the issue text.