Rework `Window` margins and set `clip_rect_margin` to zero

[emilk]

• Follows https://github.com/emilk/egui/pull/7722
• Part of https://github.com/emilk/egui/issues/5605
• Closes https://github.com/emilk/egui/issues/3385

Now that we have ScrollArea::content_margin, we should not need clip_rect_margin.

[github-actions[bot]]

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