This is a boolean attribute which corresponds to the propborderless flag of
the propinfo structure's flags field (defined in <intuition/intuition.h>).
If this attribute is TRUE, the new propgclass object will not have a
border around it.  In an autoknob propgclass gadget, if the propnewlook
flag is set as well (see the pga_newlook attribute), the knob will have a
3D appearance.