* dfeet/DFeetApp.py(__init__.py): hookup the execute_method action
  (introspect_node_selected_cb): set execute button sensitive if a method is
  selected
  (execute_current_method_cb): get the current selected method and busname and
  bring up the execute dialog
* dfeet/_ui/busbox.py: Add introspectnode-selected signal for when a node is
  selected by the BusNameInfoBox child
  (busname_info_selected_cb): emit introspectnode-selected when the BusNameInfoBox
  child emits a selected signal
  (get_selected_introspect_node): returns the current selected node of the
  BusNameInfoBox child
  (get_selected_busname): returns the currently selected busname in the
  BusNameBox
* dfeet/_ui/busnameinfobox.py: new selected signal
  (__init__): add pixbuf renderer to the treeview
  (cursor_changed_handler): emit selected
  (get_selected_node): returns the current selected node
* dfeet/introspect_data.py(Node.get_icon_name): default to None
  (Method.get_icon_name): return 'gtk-execute' for now
  (IntrospectData): add new icon_name column