ScrollablePage: Port sizing to more conventional expressions and ?. operator

Modern JS nullish-coalescing and optional chaining operators let us
write shorter and subjectively more readable code.

Regarding sizing, they were both far from conventional bindings. The
implicitHeight got ported to relatively new-ish all-in-one
implicitHeaderWidth/implicitFooterWidth properties, which it couldn't
use before because of import version restrictions. The implicitWidth
just omitted a lot of components for seemingly no reason.

I wonder if contentHeight should fallback to `undefined` to reset it
(QQuickPane supports that).