This patch adds toolbar tabs as proposed in T37601
- panels define own categories, draw as needed.
- panel pinning
- uses theme colors
- uses language translations
- scales with DPI / retina display / panel zoom.
- Hide tab-bar when no tabs/panels are visible (or only one tab would be visible).
- Tabs scale to fit smaller spaces.
- Right mouse menu on header.
- create useful panel groupings (just in python files)
Areas to check on:
- tab order? - currently same as panel draw order.
- method of making space for the tabs could work in view2d code like scrollbars currently do.