Minimal RetroLab support

[bollwyvl]

This adds fairly bare-bones support (no custom diagram launcher, status bar replacements) for RetroLab.

references

• fixes #65

changes

• [x] add retro to various envs
• [x] update core plugin
  • [x] make ILayoutRestorer optional
  • [x] add Export Diagram As FORMAT to File Menu
• [x] add to tree
  • [x] add to file browser menu bar, (gave up on custom because new window)
• [x] add to edit
  • [x] hack css for full width (gave up on height, guess there's zen mode)
  • [x] hack favicon
• [x] bail on notebook
• [x] add to robot tests
• [x] add to docs

screens

tree	edit

[codecov[bot]]

Codecov Report

Merging #72 (3182869) into master (74ac722) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #72   +/-   ##
=======================================
  Coverage   96.47%   96.47%           
=======================================
  Files          25       25           
  Lines         710      710           
  Branches       58       58           
=======================================
  Hits          685      685           
  Misses         13       13           
  Partials       12       12           

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