Gets the class group name from the <ulink url="http://tronche.
com/gui/x/icccm/sec-4.html#WM_CLASS">WM_CLASS Property</ulink> for this.
The class group name is also the identifier name of the ClassGroup to which this belongs.
| this |
a Window. |
|
the class group name for this, or null if this belongs to no class group. |