ENH: expose loadable module displayable managers to python

Before this change, you could get at module displayable managers but not
call any of their specific methods in python. This change follows the
pattern of the module logic init file.

To get at the module displayable managers for the first 3D window:
lm = slicer.app.layoutManager()
td = lm.threeDWidget(0)
dms = td.getDisplayableManagers()

The markups fiducial displayable manager was at index 7 in my test:
mfdm = dms.GetItemAsObject(7)
h = mfdm.GetHelper()
from there you can debug the seed widgets

Annotation, Volume Rendering and Tractography displayable managers
are now also accessible from python.