Restore Context of opened Nuke Script properly

[RicardoMusch]

This PR solves the following issue with the current tk-nuke implementation.

Common situation (example)

• Path to Nuke Workfile usually includes the Shot, Step, filename but NOT the Task name/id.
• Nuke Users prefer to load recent files from the File --> Recent Files menu.
• tk-nuke attempts to create the current context from the opened Nuke script file which usually results in an incomplete context (usually missing Task).

Result: Current context is set to Shot --> Step but not Shot --> Step --> Task.

Solution (this PR):

• During the onScriptSave callback of the tk-nuke engine we ensure that a tab exists on the Root node called sgtk and a string knob exists called sgtk_context.
• During onScriptSave we serialize the current context (which likely includes the Task) and set this on the sgtk_context knob.
• During sgtk_on_load_callback we attempt to read a serialized context string from the sgtk_context knob on the root node sgtk tab.
• If found, we deserialize the context and refresh the engine with this context.
• Should we fail to gain a context this way the engine will fall back to it's current implementation (context_from_path).

All of the above gracefully handles the tab or knob not being present or empty.

The behaviour in this PR is entirely nuke-native and also how other software products, such as Deadline for example, save script based properties.

I can not think of any reason this PR would break anyone's current workflow, it simply solves and annoying "shortfall" of the tk-nuke engine.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 20.14%. Comparing base (e270632) to head (c1ca408).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   19.04%   20.14%   +1.09%     
==========================================
  Files           6        6              
  Lines         546      546              
==========================================
+ Hits          104      110       +6     
+ Misses        442      436       -6     

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