Saved queries section

It would be great to create special section with saved queries. Every potential use-case of the app should be possible to have a query, which represents it.

For instance, a user learns foreign language. She might have various resources in several roots: screenshots with new words from various dictionaries, books as PDF files, links to the internet, etc. So the query could be `"spanish, study" OR "spanish, words"` and folders: `"screenshots", "documents", "bookmarks"`.

Saved queries should have their own screen. Each query should have preference regarding which query mode to use for the query: `focus`, `intersection` or `union`. Each query also could have specified tags to filter out.

Additional sub-features would be nice to have:
- ARK-Builders/ARK-Navigator#79
- ARK-Builders/ARK-Navigator#360
- Ability to override global preferences for a query
- Ability to create new query from selected tags in tag selector
- Ability to specify tags to filter-out

No files, tests, or entry points are named. Start by surveying the existing query, tag-selector, and preference screens, then define the saved-query screen, query modes, and per-query filtering and preference behavior. Done means the listed saved-query use cases and additional sub-features have clear, working coverage.

Written by the indexing model from the issue text.