Add mouse_events option on the group level

Add mouse_events option on the group level

No runtime changes. This makes it possible to set mouse_events on a
group, and it will use this value as the default for mouse_events on
every part of that group. It defaults to 1 if not set, to keep things
working as they do now.

That is, instead of setting mouse_events: 0; on every single part you
don't want to receive events, it's now possible to set it for the group
and only use mouse_events: 1; explicitly on those that should handle
events.