Backport PR #5458: Add support for PyQt5.

This PR adds the new `QT_API_PYQT5` to `IPython/external/qt_loaders.py ` and allows to embed the IPython console in Qt5 Applications. In case of PyQt5, the loader returns a union of `PyQt5.QtGui` and `PyQt5.QtWidgets` acting as a Qt4-QtGui compatibility module.

One small fix to `InProcessChannel.__init__` was necessary though. For some reason, when constructing `QtInProcessShellChannel`, the `QtCore.QObject.__init__` initializer from `SuperQObject` indirectly calls the `InProcessChannel.__init__` function, which doesn't have a default constructor and thus fails. I assume this has something to do with the metaclass/multiple inheritance approach IPython uses on QObject. Adding a default value for `client` in `InProcessChannel.__init__` solved the problem for me.