Context menu should be displayed by clicking at a tag

[kirillt]

We should be able to run various actions involving tags, e.g. #49, #50.
The list of actions can be shown via context menu:

1. In Gallery screen we would just call it by single-tap.
   Current remove tag from resource action (triggered by long-tap) must be moved into the menu.

2. We need to be able to call the menu from tag selector (in resource grid).
   Single-tap still should be handled as inclusion of clicked tag into the query.
   Double-tap must spawn context menu with options including inverted selection.

[kirillt]

#196 should be also added into Gallery's context menu