Fix issues with desired size system

[StarryWorm]

What problem(s) does this PR solve?

• Closes Label with autowrapping enabled will not automatically expand its height when its width is decreased #119325
• Closes BoxContainer, GridContainer, and FlowContainer don't respect their children's desired sizes without SIZE_EXPAND flag #119346
• Bugsquad edit: closes [4.7.beta2] Crash when adding a sibling to Label using autowrapping with a maximum size parent #120099.

Fixes the issue whereby a Label using custom_maximum_size instead of custom_minimum_size to define its behavior would not expand vertically when shrunk horizontally whilst in autowrapping mode

Before	After

Test Project (from #119325):
no-clip-label-mrp.zip

Fixes the issue whereby layout Container types, namely BoxContainer, GridContainer, and FlowContainer, would not allocate size to their children that have a desired size greater than their minimum size.

Layout computation flow:

1. Give each child its minimum size
2. From the remaining space, distribute to children with a desired size greater than their minimum size based on the ratio of their desired sizes (i.e. a child who desires twice as much space as another will get twice as much, regardless of how much they end up getting).
3. If any, from the remaining space, distribute to children with the SIZE_EXPAND flag based on their stretch ratios.

Step 2 is new, everything else stays unchanged.
In the case of FlowContainer, this process is actually much simpler, as we can just swap the desired size in for the minimum size, and then trim it back if there isn't enough space to fit it alone on a line/column.

Test Projects:

1. For a simple Label (from BoxContainer, GridContainer, and FlowContainer don't respect their children's desired sizes without SIZE_EXPAND flag #119346): container-desired-sizes-mrp.zip
2. For a layout with SIZE_EXPAND child: respect-desired-sizes-mrp.zip

Additional information

Bundled as a single PR since it's all related to the same system. Split commits for now, can squash if preferred.

Discovered this actually didn't need a more thorough rewrite of the sizing system, so getting it out now for maybe 4.7.1 cherrypicking.

[StarryWorm]

It appears this also fixes #120099, want someone else to double-check before I mark it as such in PR description

[YeldhamDev]

Looks fine and fixes the issues. But I would still like a second review for this.

[KoBeWi]

Can confirm it fixes the linked issues. The code seems fine overall.