Page: Fix title delegate elision glitch

Implicitly sized items like QtQuick/Text don't play nicely with Loader,
and generally with kinda-recursive bindings on Layout.* properties.

This combination of two fixes does the trick:

1. Use extra TextMetrics for reliable width/height values.
2. Round up text's advance width, so that container loader or layout
won't ever round it down (which it did with implicitWidth before).