ewmh.SupportingWmCheckSet should set property on both root and window

[dominikh]

_NET_SUPPORTING_WM_CHECK needs to be set to the ID of the dummy window for both the root and the dummy window itself. Currently, it's only being set for the root window.

The Window Manager MUST set this property on the root window to be the ID of a child window created by himself, to indicate that a compliant window manager is active. The child window MUST also have the _NET_SUPPORTING_WM_CHECK property set to the ID of the child window

One could argue that the user should simply do two calls to the function; alternatively one could argue that the only use of that function is to set it for both windows.