Header/include cleanups across the codebase.

* Eliminate all instances of including QtCore, QtGui, or QtXml.
* Replace all old-style Qt header inclusions with new-style (e.g. #include <application.h>).
* Include QtDebug instead of QDebug.
* Update many classes to include the Qt classes they use in their header and implementation file.

This speeds up compile and link performance and reduces the size of the compiled object files. My build directory dropped by 50MB.