fix: add deprecated QStyleOption names in the Qt5 interfaces

[grlee77]

This PR resolves a crash in Mayavi when using pyface 6.0 and PyQt 5.6.

The crash can be reproduced by trying to drag the Tab labeled "Mayavi Scene 1" in the screen shot below:

Trying to drag the tab crashes the application with the following traceback.

Traceback (most recent call last):
  File "/Users/lee8rx/anaconda/lib/python3.5/site-packages/pyface/ui/qt4/workbench/split_tab_widget.py", line 897, in mouseDoubleClickEvent
    self._resize_title_edit_to_current_tab()
  File "/Users/lee8rx/anaconda/lib/python3.5/site-packages/pyface/ui/qt4/workbench/split_tab_widget.py", line 985, in _resize_title_edit_to_current_tab
    tab = QtGui.QStyleOptionTabV3()
AttributeError: module 'pyface.qt.QtGui' has no attribute 'QStyleOptionTabV3'

The names QStyleOptionTabV3, QStyleOptionTabV2 and QStyleOptionTabBarBaseV2 have been deprecated and do not exist in the PyQt5 or PySide2 namespaces. This PR adds them to the pyface.qt.QtGui namespace. Relevant Qt documentation:
https://doc.qt.io/qt-5.10/qstyleoptiontab-obsolete.html
https://doc.qt.io/qt-5/qstyleoptiontabbarbase-obsolete.html

[codecov-io]

Codecov Report

Merging #323 into master will increase coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
+ Coverage   24.63%   24.65%   +0.02%     
==========================================
  Files         455      455              
  Lines       25541    25547       +6     
  Branches     3851     3851              
==========================================
+ Hits         6291     6298       +7     
- Misses      18936    18937       +1     
+ Partials      314      312       -2

Impacted Files	Coverage Δ
pyface/qt/QtGui.py	52.38% <50%> (-0.96%)	⬇️
pyface/ui/qt4/util/modal_dialog_tester.py	89.47% <0%> (+3%)	⬆️

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[corranwebster]

Thanks!