Fix TraitsUI dependency of GuiTestAssistant

[corranwebster]

Checks to see if traitsui is available before patching StickyDialog.

Fixes #341 and fixes #353.

[corranwebster]

Hmmm... makes sense to fold in a fix to #353 into this PR as well.

[codecov-io]

Codecov Report

Merging #366 into master will increase coverage by 1.17%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
+ Coverage   35.15%   36.32%   +1.17%     
==========================================
  Files         463      481      +18     
  Lines       25820    26406     +586     
  Branches     3838     3911      +73     
==========================================
+ Hits         9076     9591     +515     
- Misses      16350    16401      +51     
- Partials      394      414      +20

Impacted Files	Coverage Δ
pyface/ui/qt4/util/gui_test_assistant.py	80.18% <75%> (+1.53%)	⬆️
pyface/action/api.py	100% <0%> (ø)	⬆️
pyface/timer/api.py	100% <0%> (ø)	⬆️
pyface/ui/wx/fields/combo_field.py	75.47% <0%> (ø)
pyface/ui/wx/fields/field.py	81.81% <0%> (ø)
pyface/ui/qt4/fields/combo_field.py	79.24% <0%> (ø)
pyface/action/traitsui_widget_action.py	100% <0%> (ø)
pyface/ui/wx/fields/text_field.py	81.39% <0%> (ø)
pyface/fields/combo_field.py	100% <0%> (ø)
pyface/ui/qt4/fields/spin_field.py	100% <0%> (ø)
... and 25 more

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[jwiggins]

LGTM