Fix several sizing bugs

[dhardy]

Use updated kas-text, allowing use of measure_height with a line limit. Fixes: SelectableText and EditField can more accurately set their minimum sizes based on text, just dpem.

Opinionated fix: let Stack infer its minimum size only from the active page. Other size rules (ideal, margin, stretch) all infer from the max of used pages. This has two implications:

1. A Stack will still try to be large enough for all pages (up to the size limit), but may now be smaller than the largest minimum.
2. A Stack forcibly sized below its minimum allows the active page to be shrunk correctly. Without this change, the active page could be larger than the widget even if it could actually be sized smaller.

Fix: update window size requirements after resizing. This allows height-from-width requirements to be updated which may force a window to be taller when made narrower. (This was changed in #588 and is what allowed the gallery's Stack to be sized too small.)