UI/Qt: Handle pinch zoom gestures

Translate Qt native zoom gestures into Web::PinchEvent for the web
content view. This gives the Qt UI the same pinch-to-zoom input path as
the AppKit UI while leaving ordinary wheel scrolling unchanged.

Map the Qt native gesture screen position back into WebContentView
before creating the event, since macOS delivers the gesture through the
top-level widget window after forwarding. Keeping the pinch focal point
in content coordinates lets visual viewport zoom preserve the user's
focus point.