The plugin grab APIs are a bit weird right now, and they duplicate functionality provided by signals for input handling. It might make more sense to remove the grab API and instead use the raw events + make sure that plugins can block all handling without listening to all possible input events.

The input capability should however still be required for doing this, in order to ensure that a single plugin holds such an exclusive grab at a time.

This can be easily done by moving grabs to be scene nodes above the other nodes. We need to be careful with grabs like move, which need to get a release event without the corresponding press (needs to be special cased in the focus hand-off)