Add support to `tags:` scheme in DolphinQuery

It allows to start a search from a tags:/mytag/ url.

The use case is to navigate through the tags:/ slave, and then open search to refine on additional terms (another tags, ratings, etc). Currently, opening search from here does nothing.

This workflow has been suggested on https://bugs.kde.org/show_bug.cgi?id=356062#c7 (wrong place, but valid case)

Depends on: !23 (merged)