dgui: store guiItems on ShowBase class dict instead of module

direct/src/gui/DirectGuiBase.py

@@ -81,7 +81,7 @@ __all__ = ['DirectGuiBase', 'DirectGuiWidget']
 
 from panda3d.core import *
 from direct.showbase import ShowBaseGlobal
-from direct.showbase import ShowBase
+from direct.showbase.ShowBase import ShowBase
 from . import DirectGuiGlobals as DGG
 from .OnscreenText import *
 from .OnscreenGeom import *