Resizing a rectangle or ellipse while keeping its proportions (a circle
that stays a circle, a 16:9 card that stays 16:9) needed W and H typed
or keyed separately. The Timeline's Size row of a shape's Rectangle
Path and Ellipse Path now has the chain link that Scale and Mask
Feather have. It starts off; turned on, typing or scrubbing W or H
keeps the ratio (Alt still edits one value).
Scale and Mask Feather keep their linked-by-default state in `unlinked`;
Size, which starts unlinked, records a turned-on link in a new `linked`
set, so neither default changes the other's saved state.