cxx/gui: add explicit initialization of Qt resources

This adds a possibility to explicitly initialize Qt resources from the shared library. As explained in #215 (closed) in principle this should happen automatically, but for some reason (symbol visibility, treatment of unused symbols by the linker, initialization of static members in shared libraries...) this does not seem to always happen.

Now, an explicit call to

#include "constellation/gui/qt_utils.hpp"

int main() {
    constellation::gui::initResources();
}

loads the symbols. The macro needs to be called outside any namespace, hence the static dance.

This closes #215 (closed)