[Xcb] Add X Generic Extension event support

[Daniel Martin]

Hi everyone,

fixing the full_sequence problem from the ground up would have been a
nice thing. But, I got conviced that this is a way we can't go without
causing a lot of trouble. So, we've to deal with it and that's what
these two patches do.

The first one is for the proto. It introduces a new boolean attribute
"xge" to events. If it's set to "true", then xcbgen generates the XGE
event header:
    [PATCH proto] Add support for X Generic Extension events

The second for the lib injects the full_sequence into XGE events at the
32byte boundary. That wouldn't be necessary for XGE events <= 32bytes,
but it'll be injected in every XGE event for convenience:
    [PATCH libxcb] c_client.py: Inject full_sequence into GE events

Cheers,
    Daniel Martin

PS: I've tested it with a few XI2 events:
        https://github.com/bartsch/xcb-proto/tree/rev-xinput