OOUI PHP: Use 'OOUI' namespace instead of 'OoUi' class name prefix

* Added 'namespace OOUI;' and removed 'OoUi' class prefixes from all
  PHP source files with simple search-and-replace.
* Renamed the files accordingly.
* Updated Doxyfile, I hope this works like I think it does.
* Updated widgets demo and comparison test runner.

Bug: 72705
Change-Id: I79bcd944859f8b2f1d85da96c28e83c8d04a368a
More random documentation fixes

Change-Id: Id62049cb00c04db5579646e66973ca41029e11af
Merge "OOUI PHP: Use 'OOUI' namespace instead of 'OoUi' class name prefix"
Merge "More random documentation fixes"