Sets hints about the window management functions to make available via buttons on the window frame.
On the X backend, this function sets the traditional Motif window manager hint for this purpose. However, few window managers do anything reliable or interesting with this hint. Many ignore it entirely.
The functions argument is the logical OR of values from the GdkWMFunction enumeration. If the bitmask includes GDK_FUNC_ALL, then the other bits indicate which functions to disable; if it doesn’t include GDK_FUNC_ALL, it indicates which functions to enable.