[Xcb] Please review/comment: libxcursor port

[Bastien Dejean]

Michael Stapelberg:

> Also it would be cool if you could test it and see if it works with
> your favorite cursor theme, in case you use any ;-):

Seems to work with the cursor theme I currently use (dmz-aa).

Some windows won't let their cursor be changed though (e.g.: URxvt), is
this a known limitation?

I was also wondering if it might be possible to retrieve the current
cursor for a given window in order to restore it afterwards?