Python binding was broke since commit b1f37a8.
This is a fix and to some degree a refactor.
The binding build is now integrated. Controlled by the CMake
QTERMWIDGET_BUILD_PYTHON_BINDING option.
CMake components taken from https://cgit.kde.org/pykde5.git/. No need to
reinvent the wheel.
Closes https://github.com/lxqt/qtermwidget/issues/135.
Alf Gaida