Tags editing dialog: quick tags and filter by prefix

[kirillt]

Would be nice to add additional selector in tags editing dialog, used for putting tags into input field.
This new selector should be located under text input field and consist of the most frequent tags across the whole storage.

When a user types text into the field, quick tags must be filtered by prefix. This filter must be reset upon entering the tags separator (comma).

[kirillt]

This new selector should ... consist of the most frequent tags across the whole storage.

The simplest way is to make it contain only the most popular tags (the more resources a tag contains the more popular it is).

Ideally, it also should contain the most recently used tags. We need to maintain stack of tags in Room for this.
Every time a tag is used, it is placed on top of the stack. We keep maximum K tags in the stack.

Quick-tags selector would contain several tags from both the most popular and the most recent, but totaling N tags, where N > K: take the most recent tags first (from 0 to K), fill the rest with the most popular tags (from N-K to N).

[kirillt]

See point 4 in #16 for definition of "recently used tags".

[kirillt]

Moving the most recently used tags into separate issue: #25

[kirillt]

Follow-up: #26