Remove include of QtGui in favor of individual includes

The reason for removing instances of "#include <QtGui>" is twofold:

1) Including it will probably include more than is actually needed. Plus,
   it hide what is actually needed, especially if in a header

2) Qt 5 splits Gui into Gui and Widgets, so being explict about what is
   needed gets around this problem
Merge pull request #1245 from simonsonc/remove-qtgui

Remove include of QtGui in favor of individual includes