Palette shortcuts

[VanSHOE]

This is a proof-of-concept Pull Request containing all the work done on palette shortcuts done up till now.
After discussion with @shoogle, we found some issues that would require to be solved before this system can be implemented.
I mentioned these issues in GSoC'22 Week 8 blogpost.
This particular feature spans months of work hence there might be some useless pieces of code that I will clean up.

Currently you can assign shortcuts to palette items from the palette sidebar or from preferences.
The shortcuts are stored in the workspace where other palette cell data is stored.

• I signed CLA
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• I created the test (mtest, vtest, script test) to verify the changes I made