Bitcoin-Qt: massive header and cpp cleanup

This pull is currently here to see how pull-tester is behaving with it. To really be sure I didn't break something this pull should be send through a Gitian build.

I tried to harmonize the layout / style in all changed files. The QApplication on top is related to Qt5 compatibility, I need to see if this is working correctly.

@laanwj I know this has the potential to make you angry or at least may steal some of your time, but in the end it should be well worth it. I even think removed includes could speed up compilation time a little :).

- try to enforce the same style to all Qt related files
- remove unneeded includes from the files
- add missing Q_OBJECT, QT_BEGIN_NAMESPACE / QT_END_NAMESPACE
- prepares for a pull-req to include Qt5 compatibility