[The Task](https://www.pivotaltracker.com/story/show/180887253)
A new (+) button on the left-bottom corner appeared. It may be clicked to open searcher in the middle of the scene, as an alternative to tab key.
https://user-images.githubusercontent.com/3919101/154514279-7972ed6a-0203-47cb-9a09-82dba948cf2f.mp4
# Important Notes
* The window_control_buttons::common was extracted to separate crate `ensogl-component-button` almost without change.
* This includes a severe refactoring of adding nodes in general in the Graph Editor. The whole responsibility of adding new nodes (and starting their editing) was moved to Graph Editor - the Project View only reacts for GE events to show searcher properly.
* The status bar was moved from the bottom-left corner to the middle-top of the scene. It does not collide with (+) button, and plays "notification" role anyway.
* The `interface` debug scene was buggy. The problem was with one expression's span-tree. When I replaced it, the scene works.
* I've removed "new searcher" API, as it is completely outdated.
* I've changed code owners of integration tests to GUI team, as it is the team writing mostly the integration tests (int rust)
