Qt 4/5 specific screen grab routines

[j0yu]

This is a squash of wwfxuk's fork's 3677f70 and e9b03bc

Qt4's QPixmap.grabWindow has been removed in Qt5 and their Qt5 Screenshot Example has changed to use QScreen.grabWindow.

Discovered with Katana 3.1 (PyQy5), this PR fixes the following error:

AttributeError: type object 'QPixmap' has no attribute 'grabWindow'

An AttributeError occurred in "screen_grab.py": type object 'QPixmap' has no attribute 'grabWindow' root    2020-04-23T15:33:37
Traceback (most recent call last):
  File "/path/to/pipe/config/install/app_store/tk-framework-widget/v0.2.7/python/thumbnail_widget/thumbnail_widget.py", line 138, in _on_camera_clicked
    pm = self._on_screenshot()
  File "/path/to/pipe/config/install/app_store/tk-framework-widget/v0.2.7/python/thumbnail_widget/thumbnail_widget.py", line 230, in _on_screenshot
    pm = screen_grab.screen_capture()
  File "/path/to/pipe/config/install/git/tk-framework-qtwidgets.git/v2.8.5+wwfx.1.0.1/python/screen_grab/screen_grab.py", line 188, in screen_capture
    pixmap = get_desktop_pixmap(tool.capture_rect)
  File "/path/to/pipe/config/install/git/tk-framework-qtwidgets.git/v2.8.5+wwfx.1.0.1/python/screen_grab/screen_grab.py", line 336, in get_desktop_pixmap
    return QtGui.QPixmap.grabWindow(
AttributeError: type object 'QPixmap' has no attribute 'grabWindow'

Also includes similar fix for Maya 2019 to #95

See also:

• Do a diff yourself between Qt5 Screenshot Example's screenshot.cpp and Qt4 Screenshot Example's screenshot.cpp
• Our internal v2.8.5+wwfx.1.0.2, v2.9.2+wwfx.1.0.1 release notes
• Original, internal PR Fix Qt5 removal of QPixmap.grabWidget wwfxuk/tk-framework-qtwidgets#4 and Fix for Maya 2019 error wwfxuk/tk-framework-qtwidgets#5

[jfboismenu]

Hi @j0yu , thank you for this. I'm going to track this JIRA so we can have a developer take a look at the fix.