Use viewcode sphinx extension

[rahulporuri]

Using this extension means that API docs now link to source code. See example below which now includes the source button.

[aaronayres35]

LGTM! I tested building the docs and it works as expected

[codecov-io]

Codecov Report

Merging #851 (040a3f3) into master (92ab736) will increase coverage by 0.14%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #851      +/-   ##
==========================================
+ Coverage   41.15%   41.29%   +0.14%     
==========================================
  Files         522      522              
  Lines       28110    28126      +16     
  Branches     4251     4253       +2     
==========================================
+ Hits        11568    11614      +46     
+ Misses      16037    16015      -22     
+ Partials      505      497       -8     

Impacted Files	Coverage Δ
pyface/action/action_event.py	100.00% <ø> (ø)
pyface/action/action_manager.py	83.92% <ø> (ø)
pyface/action/field_action.py	95.00% <ø> (ø)
pyface/tasks/action/listening_action.py	0.00% <0.00%> (ø)
pyface/tasks/action/schema.py	87.93% <ø> (ø)
pyface/tasks/action/schema_addition.py	100.00% <ø> (ø)
pyface/tasks/action/task_action.py	60.00% <ø> (ø)
pyface/tasks/action/task_action_controller.py	52.94% <ø> (ø)
pyface/tasks/action/task_action_manager_builder.py	91.96% <ø> (ø)
pyface/tasks/action/task_toggle_group.py	48.97% <ø> (ø)
... and 20 more

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