Move tool settings bar from top bar to individual editors with toolbar.
* 3D Viewport
* UV editor
The tool settings bar visibility will be opaque and above the header. It will be possible to hide just the tool settings but keep the header. In this case the editor type switching moves from the tool settings bar to the header.
The tool settings bar would contain:
* Editor type switching
* Mode and select mode
* Tool settings
* Orientation, snap, ..
* Popovers for mode panels
The header would contain
* Menus
* Display settings
Ideally quad splitting would be improved so it's easy to have e.g. two viewports with a single topbar, but this is not required for 2.80.