RFE: XGE generic event channels

The Present extension makes the notable improvement that events are selected and delivered relative to an EVENTID type - "event channel" might be a better name. This allows for multiple subsystems within a process to receive a copy of any given event, without affecting other subsystems or needing to rely on a client-side convention for event queue interest. (Well. Kinda. It bakes that convention into libxcb as "special events". Whatever.)

Unfortunately it stops there, and it doesn't really belong there. Event channels ought to be an XGE extension concept, and they should be able to encapsulate arbitrary core or extension events. This could massively improve life on the client side. A few examples off the top of my head:

• Multiple readers could listen for ConfigureNotify on the root window, or corresponding RANDR events, and all would get notified
• llvmpipe would be able to use the send_event=True version of ShmPutImage to increase parallelism
• libX11 could potentially internally redirect event selection to its own private channel, which might finally solve the "who owns the event queue" problem when mixing xcb and xlib in one process