Graph tags selector

When tags selector is maximized, tabs with alternative selectors must appear. One of alternative selectors is _cloud selector_, and other is _graph selector_. Both should switch to default _linear one_ upon dragging selector down.

_Graph selector_ represents a graph with tags as its vertices. When two tags _A_ and _B_ intersect (there are resources labeled with both of them) then we have edge between _A_ and _B_.

No files or tests are named. Start by locating the maximized tags selector and the existing linear selector, then trace how selector dragging changes the active mode. Done means maximized mode exposes cloud and graph selector tabs, and dragging either alternative selector down switches back to the default linear selector.

Written by the indexing model from the issue text.