wayland: Remove special code for modal grabs

Since we never pass any Clutter events to Wayland, it's not needed.
Add META_GRAB_OP_WAYLAND_CLIENT

Which is used for Wayland popup grabs.

The issue here is that we don't want the code that raises or focuses
windows based on mouse ops to run while a client has a grab.

We still keep the "old" grab infrastructure in place for now, but
ideally we'd replace it eventually with a better grab-op infrastructure.