Making panel_widget attribute consistent.

PropertyGroups and Properties used different attributes to specify the type of
widget to use. Changing the attribute used by PropertyGroups (type) to be same
as the other (panel_widget).

Change-Id: I2910a3de34c9bb488fd638b6e9f5ebd3f0f33014
Merge topic 'panel_cleanups' into next

3fda6c5 Making panel_widget attribute consistent.