[Xcb] Setting the WM_TRANSIENT_FOR property on a window

[Arnaud Fontaine]

    > If  I were designing this  API, I'd probably make  it the caller's
    > problem to concatenate the  two strings. Callers might hate me for
    > it, but at least I  wouldn't have to think about memory allocation
    > or whether the input is valid. ;-) Applications usually use string
    >  literals for instance  and class,  right?  It's  easy to  write a
    > literal containing both values ("instance\0class") and then nobody
    > has  to call malloc.   Even applications that use  dynamic strings
    > may be able to allocate them more efficiently than xcb-icccm can.

I have attached this patch to this email. Hope that's ok.

Arnaud