Create a mechanism to display the available objects when in interactive Python mode

[griffin28]

Allowing the user to display the list of available PyDV objects will make it easier to create custom "PyDV Macros" like below (courtesy of Rob Managan):

[PyDV]: cur a.4 b.4
[PyDV]: drop
Starting Python Console...
Ctrl-D to return to PyDV
Python 2.7.10 (default, May 26 2015, 14:17:46)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-11)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> def testFunc(line):
... print " this is a test, here is rest of line '", line, "'"
...
>>> self.do_test = testFunc
>>>
Python Data Visualizer 2.3 - 11.20.2017
Type "help" for more information.
[PyDV]: test hhh rest of input line jjj
this is a test, here is rest of line ' hhh rest of input line jjj '
[PyDV]: