Rework Window margins and set clip_rect_margin to zero

[emilk]

• Follows Add ScrollArea::content_margin #7722
• Part of Remove clip_rect_margin #5605
• Closes ScrollAreas can overrun their container's margins #3385

What

Sets clip_rect_margin to zero, and moves the margin of Windows with ScrollAreas, so that the scroll bars are now on the very edge of the windows they are in.

Required

• Fix jitter when hovering edge of scroll area close to resize splitter #7803
• Make scroll bars and resize splitters visible to accesskit #7804
• Allow moving existing widgets to the top of interaction stack #7805
• Slightly change interact behavior around thin splitters #7806
• Move window resize interaction to be over contents #7807
• Don't expand widgets on hover #7808

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7725-emilkno-clip-rect-margin
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

View snapshot changes at kitdiff

[emilk]

Oops - the windows headers shrunk… a lot. Good with screenshot tests 😅

[emilk]

Oh no… I need to do the resize-corner interaction on top of the scroll area, or they will fight with each-other

[emilk]

I also need to tweak should_prioritize_hits_on_back… or change the size of the resize corner or something.