Cleanup API, file names, documentation

[cschreib]

This PR does:

• fix formatting of some code that was missed in previous formatting PR
• fix file names to use a consistent convention for spaces
• fix spacing around colons on error messages
• fix formatting of Doxygen documentation comments
• use magic_enum for serialisation of enums
• rework API for registering region types
• rename anchor_point to point
• merge all orientation enums (gradient::, slider::, status_bar::) into a single shared one