Inheritance issue when creating GUI from Python

Inheritance of Python classes from C++ does not work in some cases, as discussed in:

https://sft.its.cern.ch/jira/browse/ROOT-10789
https://sft.its.cern.ch/jira/browse/ROOT-10582

This ticket is created to track how that issue affects the creation of GUIs from Python. In particular, the GUI example tutorial in Python does not work:

https://root.cern.ch/doc/master/gui__ex_8py.html

since it fails to inherit from TGMainFrame, with the following error:

input_line_39:9:43: error: base class '::TGMainFrame' has private copy constructor

  Dispatcher1(const Dispatcher1& other) : TGMainFrame(other), m_self(other.m_self, this) {}

                                          ^

/home/etejedor/root/fork/build/include/TGFrame.h:487:4: note: declared private here

   TGMainFrame(const TGMainFrame&);             // not implemented

   ^

a.py:2: RuntimeWarning: no python-side overrides supported

  class pMainFrame( ROOT.TGMainFrame ):

Once the issue above is fixed, in addition, the TPyDispatcher functionality used in the old PyROOT should be replaced by a better alternative to register Python callables as GUI callbacks. We should be able to do this automatically by relying on the automatic generation of C++ wrappers for Python callables that cppyy provides.