Union query mode

In addition to `intersection` and `focus` query modes, `union` mode should be added.
When `union` mode is enabled, we display all resources **marked with at least one of specified tags**.

- [ ] `Union` query mode must be implemented
- [ ] `Focus` mode should be made default query mode
- [ ] New icons for `intersection` and `union` modes

"Intersection" mode is called "normal" in ARK-Builders/ark-components-android#17.
"Focus" mode was implemented in scope of ARK-Builders/ark-components-android#23.

New icons for query modes should remind the following:

## How `union` query mode works

Suppose, we have the following resources:
```
┌────────────────┐
│ # | tags │
├────────────────┤
│ 1 | a │
│ 2 | b │
│ 3 | c │
│ 4 | a, b │
│ 5 | b, c │
│ 6 | a, c │
│ 7 | a, b, c │
│ 8 | │
└────────────────┘
```

This is how queries will work in `union` mode:
```
┌─────────────────────────────────┐
│ query | resources │
├─────────────────────────────────┤
│ {} │ │
│ {a} │ 1, 4, 6, 7 |
│ {a,b} │ 1, 2, 4, 5, 6, 7 │
│ {a, b, c} │ 1, 2, 3, 4, 5, 6, 7 |
└─────────────────────────────────┘
```

The issue names no files or tests. Start by tracing the existing intersection and focus query modes, then verify the union behavior against the resource and tag examples; done means union mode, the default focus mode, and the requested query-mode icons are implemented.

Written by the indexing model from the issue text.