Remove header bar / content separation
Currently the header bar and content areas in a window are separate, which is causing issues when trying to do more "interesting" things with header bars, such as:
- "Split header bar" layouts, such as the one in Settings are not easy to implement, which is why many third party apps do it badly (misaligned separators, no separators, etc.)
- Hiding the header bar (#2240 (closed))
- Various adaptive things libhandy is doing, such as HdyLeaflet