[Xcb] XCB convenient functions

[Vincent Torri]

Hello,

I've added some convenient functions in the cvs
(xcb-util/convenient/xcb_convenient.*)

Do you think i should change the name of the functions, so that hthey
match the XCB naming ? (like XCBConnectionDepthGet or
XCBConnectionGetDepth instead of xcb_connection_depth_get)

do you have other functions that you would like to be added ?

regards

Vincent