Save as code snippet from editor selection

[karlaspuldaro]

Resolves #1005

This PR implements the 'Save as Code Snippet' command.

1. Select text from notebook or file editor
2. Right click to open the context menu
3. Select 'Save as Code Snippet'
4. Metadata editor opens, code field is pre-populated with selection
5. User fills out remaining form fields

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[ptitzler]

Awesome. Please tag me when the feature is ready for a test drive!

[ptitzler]

LGTM. Tested different file types and various editors. One observation: it appears that the action is only enabled for "cells" that are in edit mode. For example, open a notebook with a rendered markdown cell, and select some text. The save option is disabled. Is this intentional?

[marthacryan]

Just tried locally and it worked well! +1 on Patrick's comment about not needing the cells to be in edit mode. Also, not sure if this is the desired behavior, but I think it could be good to add the behavior of saving an entire cell as a snippet if none of the text is selected.

[karlaspuldaro]

@ptitzler @marthacryan Thank you for your feedback.
I just pushed a commit addressing the selection of a rendered notebook cell and markdown preview.
I will create a follow up issue to add notebook cells as new snippet.