Re-work of TasksApplication on top of other Application PRs

[corranwebster]

This is re-work of #198 and #285 on top of #335

Needs:

• examples
• CI running correctly

[codecov-io]

Codecov Report

Merging #345 into master will increase coverage by 0.45%.
The diff coverage is 67.37%.

@@            Coverage Diff             @@
##           master     #345      +/-   ##
==========================================
+ Coverage   34.42%   34.88%   +0.45%     
==========================================
  Files         457      462       +5     
  Lines       25432    25656     +224     
  Branches     3797     3823      +26     
==========================================
+ Hits         8754     8949     +195     
- Misses      16308    16329      +21     
- Partials      370      378       +8

Impacted Files	Coverage Δ
pyface/tasks/action/listening_action.py	0% <0%> (-25.38%)	⬇️
pyface/tasks/action/task_toggle_group.py	48.97% <0%> (+0.97%)	⬆️
pyface/ui/qt4/window.py	73.87% <0%> (-0.68%)	⬇️
pyface/tasks/traits_editor.py	47.05% <0%> (-2.95%)	⬇️
pyface/action/api.py	100% <100%> (ø)	⬆️
pyface/api.py	100% <100%> (ø)	⬆️
pyface/tasks/api.py	100% <100%> (ø)	⬆️
pyface/tasks/action/task_action.py	60% <100%> (ø)	⬆️
pyface/tasks/action/api.py	100% <100%> (ø)	⬆️
pyface/ui/qt4/about_dialog.py	79.41% <28.57%> (-8.09%)	⬇️
... and 18 more

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[corranwebster]

@jonathanrocher If you would like to review this, I'd appreciate it.

[corranwebster]

At this point I am going to self-merge this. The PR was meant to unify approaches that @jonathanrocher and I had for Envisage-compatible application classes. It can be refined as time goes on.