UI: GUI: Add license headers to all GUI files.
UI: GUI: Fix include guards.
UI: GUI: Adapt to namespace convention.
UI: GUI: No indentation inside namespace.
UI: GUI: Add/remove newlines to make the code more Allman style.
UI: GUI: Reformat all documentation.
Add proper doxygen grouping with `gui`.
UI: GUI: Refactor unused variables warnings.
Merge branch 'feature/gui_cleanup' into develop