Qt4 compatibility

Mainly include fixes, and only two Qt4-specific changes (look for QT_VERSION in the changes);
The .pro is now easier to maintain and more portable, thanks to pkg-config.
Powiter compiles and launches with Qt4. I didn't notice any lost functionality.
Qt 4 compatibility is better, as long as no Qt5-specific feature is needed, since the Qt4 code base is much more mature.