Support flat/toolbox windows
A lot of apps have windows like this lately:
They are flat and always draggable.
We should support it properly. This includes:
- Header bars have no background or border
- They need a separate backdrop style as header bar isn't prominent enough (e.g. a different background color)
- It needs a .devel style since header bar has on background
- Typography has slightly transparent window, do we want this?
Relevant art: HUD panels in macOS