PR: Use "QtCompat" to handle "QHeaderView" incompatibilities and fix broken "resolve" button in "rez-gui".

[KelSolaar]

This PR addresses #849 by leveraging the vendored Qt.QtCompat module.

The branch is based on the https://github.com/KelSolaar/rez/tree/feature/python3 branch that is waiting to be merged in #850 (and should be merged first).

[KelSolaar]

This PR also fixes the broken resolve button problem reported in #852.

There are still a few late exceptions being raised but it mostly work:

Traceback (most recent call last):
  File "c:\progra~1\rez\lib\site-packages\rezgui\widgets\ViewGraphButton.py", line 46, in _contextChanged
    self.action_1.setEnabled(enable_resolve)
RuntimeError: Internal C++ object (PySide2.QtWidgets.QAction) already deleted.
Traceback (most recent call last):
  File "c:\progra~1\rez\lib\site-packages\rezgui\widgets\ViewGraphButton.py", line 46, in _contextChanged
    self.action_1.setEnabled(enable_resolve)
RuntimeError: Internal C++ object (PySide2.QtWidgets.QAction) already deleted.