Build & run with Qt4 and Qt5

- Set "QT += widgets" for Qt5 in .pro file
- Removed directory names from Qt includes
- Replaced some toAscii() with toLatin1()
- Added an #ifdef for getting "DocumentsLocation"
- Made createQSettings() work with Qt5

Note that menu integration in Ubuntu will only work with Ubuntu's
patched Qt version. That's the case for Qt4 and Qt5.