Create pyside2qmlplugin

Now that QTCREATORBUG-23679 is merged I'm willing to make another step towards better integration between PySide2 and Qt tools (like qmlscene, qmlplugindump, qmltestrunner and other).

In PyQt5 there is a nice plugin for that - pyqt5qmlplugin.

I already have a working plugin with similar features for PySide2. I'm calling it pyside2qmlplugin. And I would like to contribute it to PySide2 project.

Once merged, this plugin will make it possible to:

• quickly prototype QML components with custom python types and qmlscene;
• use qmlplugindump to generate .qmltypes files for python-based QQmlExtensionPlugin;
• use qmltestrunner in order to run automated tests on python-based QQmlExtensionPlugin modules.

NOTE: In addition to plugin itself, I'm also willing to add pyside2-qmlscene, pyside2-qmlplugindump and a couple of other useful python scripts to pyside/pyside-tools.

Side effect: It will be possible to use this plugin in order to integrate python-based {{QQmlExtensionPlugin}}s into C/C++ applications. This is a pretty cool feature for those who want to develop a big app (like IDE) in C/C++ and then give users an option to extend their app with python plugins.