Add Camera FPS plugin

[iche033]

🎉 New feature

Summary

Adds a simple camera FPS plugin to show the GUI camera's framerate

The background is made transparent so you can configure it to be floating on top of the 3d window, e.g.

Note: It's probably more accurate to put the logic for computing the FPS values inside the Minimal Scene plugin but I created a new Camera FPS plugin for modularity. In the Camera FPS plugin, technically it's measuring the rate at which render events are fired by the Minimal Scene plugin which happens after each GUI camera render update. I measured the difference between computing the FPS from inside the Minimal Scene plugin and also here in the Camera FPS plugin and found the difference to be < 1e-2 which is somewhat acceptable.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #523 (9f34f35) into ign-gui6 (eef51bf) will not change coverage.
The diff coverage is n/a.

❗ Current head 9f34f35 differs from pull request most recent head 22c611f. Consider uploading reports for the commit 22c611f to get more accurate results

@@            Coverage Diff            @@
##           ign-gui6     #523   +/-   ##
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  Coverage     68.06%   68.06%           
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  Files            39       39           
  Lines          5327     5327           
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  Hits           3626     3626           
  Misses         1701     1701           

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