Wasm Accessibility: Support Geometry Updates

Accessibility geometry updates are usually polling-based, where the native a11y implementation requests accessibility element geometry (via QAccessibleInterface::rect()) when required.

However, the WebAssembly accessibility backend creates an intermediate accessibility tree using DOM UI elements which needs to be updated when the Qt UI element geometry changes.

To support this Qt should push geometry updates using QAccessible::LocationChange events, and the wasm a11y backend should handle those.