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.