14.1.9 Setting and Reading the WM_TRANSIENT_FOR Property
Xlib provides functions that you can use to set and read
the WM_TRANSIENT_FOR property for a given window.
To set a window's WM_TRANSIENT_FOR property, use
XSetTransientForHint().
To read a window's WM_TRANSIENT_FOR property, use
XGetTransientForHint().
Christophe Tronche, ch@tronche.com