Union query mode

[kirillt]

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-android#17.
"Focus" mode was implemented in scope of ARK-Builders/ark-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 |  <none>    │
└────────────────┘

This is how queries will work in union mode:

┌─────────────────────────────────┐
│ query     | resources           │
├─────────────────────────────────┤
│ {}        │ <none>              │
│ {a}       │ 1, 4, 6, 7          |
│ {a,b}     │ 1, 2, 4, 5, 6, 7    │
│ {a, b, c} │ 1, 2, 3, 4, 5, 6, 7 |
└─────────────────────────────────┘