mcp: add wheel events to the tool surface

A client could move, click and drag the pointer but never scroll it, so a
Flickable or ScrollView couldn't be scrolled and a wheel-driven canvas zoom
wasn't reachable at all. Working around it meant finding the application's own
scroll or zoom controls in the element tree and clicking them repeatedly.

`ElementHandle::scroll` already dispatches `PointerScrolled` at an element's
center, so the two tools are thin: `scroll_element` takes a handle and
`dispatch_pointer_scroll` an arbitrary point, mirroring the existing
`hover_element` and `move_pointer` pair.

ChangeLog: The MCP server has new `scroll_element` and `dispatch_pointer_scroll` tools, to send mouse wheel events.