Initial Qt5 sources for Gui

[YakoYakoYokuYoku]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

See #698.

Show a few screenshots (if this is a visual change)

N/A.

Have you tested your changes (if applicable)? If so, how?

See #698.

Futher details of this pull request

Used the following command to generate the sources:

$ shiboken2 --enable-parent-ctor-heuristic --use-isnull-as-nb_nonzero --avoid-protected-hack \
      --enable-pyside-extensions --include-paths=.:Global:Engine:Gui:libs/OpenFX/include \
      --include-paths=/usr/include/PySide2:/usr/include/QtCore:/usr/include/QtGui:/usr/include/QtWidgets \
      --include-paths=/usr/include/python3.9 --typesystem-paths=Engine:/usr/share/PySide2/typesystems \
      --output-directory=Gui/Qt5 Gui/PySide2_Gui_Python.h Gui/typesystem_natronGui.xml