Previously, ListEditWidget expects values to be strings with a valid

separator, thus it can only be used to edit a list of strings.

The corresponding Django template and Backbone view are hardcoded to

display string values with text inputs.
This change modifies ListEditWidget so that it can modify a list of 

any type of values, not just strings. This change includes the creation

of a ListEditField which has the ListEditWidget as its default widget.

This field splits a string into a list of strings, reproducing the

behaviour of the old ListEditWidget. We also introduce a

ListEditDictionaryField, which takes a dictionary and splits it into a

list of (key, value) tuples.

Wrote tests for ListEditField and ListEditDictionaryField. Ran all

  tests in djblets.forms.tests with success.
Updated tests for ListEditWidget and ListEditView. Ran all Javascript Unit

  tests with success.
Performed manual tests.