Feature: UI for locking canvas size aspect ratio

[ShirleyNekoDev]

closes #1125

This PR builds upon #1171

[leMaik]

Wow, the UI for this is amazing! 🤩

[ShirleyNekoDev]

~ fixed wrongly copy-pasted copyright notice

[ShirleyNekoDev]

~ rebased onto master

[ShirleyNekoDev]

~ rebased onto master (and messed up git on the way)

[leMaik]

minor bug: one can enter a 0 in the image dimensions, which causes an exception and somehow the validation thinks it's fine

I think the ratio/resize calculation has rounding issues.

1. enter 1920 x 1080
2. apply, lock ratio
3. enter 2160 as height
   → expected: width updates accordingly, ratio 16:9
   → actual: 💥 width somehow becomes 4.330 (yes, with a comma), ratio is now 433 : 216

[Peregrine05]

Is this a problem for this PR?

As you can see, the resolution in the resolution boxes of the scene tab does not match that of the render preview.
My default resolution is set to 960x600, but the resolution boxes always display 400x400, regardless of the default or the actual setting.

[leMaik]

@Peregrine05 Can you create a new issue, please?

[Peregrine05]

All right.