v0.4.0

What's new

• Add a box model view with element bounds, content size, margin, border and padding
• Refresh the basic example with a richer layout for visual inspection

Fixes

• Prevent the picker highlight from drawing over the DevTools panel
• Clear the picker highlight immediately after selecting an element
• Remove the trailing separator from the last style row

gpui-devtools 0.4 targets gpui-pre 0.3.