InlineMessage: Fix glitched size

This patch pulls readonly properties together, and introduces some
states to manage anchor changes instead of raw bindings. One such set
of states fixes a very specific glitch in Qt 6 TextArea which may get
stuck during first resize event but fix-up itself later.

See https://bugreports.qt.io/browse/QTBUG-117667

Test plan:

- Open Kirigami Gallery, Inline Messages page; shrink window until the
  last message with action buttons wraps.
- Launch Spectacle; save a screenshot to get "The screenshot was saved
  as" inline message on top; buttons should not overlap of clip away.