demo/xcb_cairo/nuklear_xcb.h: do not discard xcb_client_message_event_t

[Shamar]

Currently nk_xcb_handle_event discard everything that was sent from other clients through xcb_send_event.

To enable a minimal IPC over such channel, this patch adds a new option NK_XCB_CLIENT_MESSAGE to the nk_xcb_event_type enum and stores the message recieved in xcb_ctx->last_client_message.

This can be useful in particular when a background process or thread completes a data retrieval or processing and want to wake up the GUI.