* Don't include <QtGui>, which is a leftover from Qt4 and causes literally
everything to be pulled into the TU.
* Use forward declarations instead of includes where possible.
* Replace aggregated Ui* classes with aggregation via pointer, which allows
these classes to be forward-declared and thus further reduce the number of
included headers. Aggregation via pointer has been the default in Qt Creator
for some time now.
* Fix minor warnings reported by Clang.
* Move QnCheckBoxedHeaderView to a separate file.
* Move QtvCodecInfo to a separate file, change initializer list to inline member initialization.
* Create an actual QMake .pro file instead of stuffing it in MXE build scripts.
* Fix 32-bit integers being used as pointers to QtvCodecInfo objects in 64-bit Windows builds.
* Update information in the "About" tab.
