Deriving tags by folder names

It is convenient to automatically create tags for roots and favorite folders.
And also label all resources from such folder with its tag.

Example:
```
Pictures
\---Cats
|---a.png
|---b.png
|---c.png
\---Dogs
|---x.jpg
|---y.jpg
|---z.jpg
```

Here we want to automatically label `a.png`,`b.png`,`c.png` with `cats` and `x.jpg`,`y.jpg`,`z.jpg` with `dogs`.
It's also would be nice to allow user to rename automatically created tags, if she prefers `dog` over `dogs`.

This feature is useful when a user has resources belonging to one category in separate roots.
Should be possible to turn on/off by folder.

Start by tracing how roots, favorite folders, resources, and tags are currently represented and updated. Define the behavior for deriving tags, applying them to descendants, renaming generated tags, and enabling or disabling the behavior per folder; done means these cases are implemented and covered by the project's relevant tests.

Written by the indexing model from the issue text.