visible_bounds operation for Container

[hecrj]

This PR introduces a brand new visible_bounds operation to container which can be used to obtain the visible screen bounds of a Container widget with an Id.

This operation can be very useful when combined with the window::screenshot command.

2023-07-27.01-20-41.mp4

Closes #1911.

[bungoboingo]

Looks good, tested it in an example with screenshots and bounds all seem correct, in scrollables too! 🥳 Only issue with the workflow is that Screenshot's size is in physical pixels bc it's based off the underlying texture size, and this operation returns logical. I think the best way to handle it would be to include the scale_factor in the Screenshot data, and then for the crop method just do the scaling for the user. I'll pop up a PR for it now.

Edit: Actually after thinking about it more, maybe the user would want to choose to use scale_factor or not in crop()? Maybe we should just include it in Screenshot and let the user handle it.

[hecrj]

I think including scale_factor in Screenshot makes sense. Everything in iced is in logical units.