Toggle grayscale of all videos using "Toggle Grayscale" button

[roomrys]

Description

Previous the "Toggle Grayscale" button in the GUI toggled the grayscale of videos on a video-by-video basis. However, this does not make sense since all projects (at the current SLEAP state) need to be either grayscale projects or color projects (see: #759). This PR broadens the functionality of the "Toggle Grayscale" button to toggle all videos to grayscale/color at once

Types of changes

• Bugfix
• New feature
• Refactor / Code style update (no logical changes)
• Build / CI changes
• Documentation Update
• Other (explain)

Does this address any currently open issues?

[list open issues here]

Outside contributors checklist

• Review the guidelines for contributing to this repository
• Read and sign the CLA and add yourself to the authors list
• Make sure you are making a pull request against the develop branch (not main). Also you should start your branch off develop
• Add tests that prove your fix is effective or that your feature works
• Add necessary documentation (if appropriate)

Thank you for contributing to SLEAP!

❤️

[codecov]

Codecov Report

Merging #1215 (b2ddca3) into develop (4bd0171) will decrease coverage by 0.02%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           develop    #1215      +/-   ##
===========================================
- Coverage    72.13%   72.11%   -0.02%     
===========================================
  Files          132      132              
  Lines        23526    23535       +9     
===========================================
+ Hits         16971    16973       +2     
- Misses        6555     6562       +7     

Impacted Files	Coverage Δ
sleap/gui/commands.py	60.63% <50.00%> (-0.23%)	⬇️

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