Implement Actions and Events as per ODK XForm Specification

[abbyad]

As a way to solve #4105, @lognaturel mentioned the Actions and Events from the ODK XForm specification. ODK Collect currently implements the xforms-ready event only, and will soon support xforms-value-changed as well. Neither are currently implemented in Enketo. For the purposes of have variable defaults we would need both of these actions.

Here is a simple form which uses parts of ODK's examples. Since events and actions are not yet supported by pyxform changes were made manually in XForm.

[alxndrsn]

Asked on ODK slack at https://opendatakit.slack.com/archives/C3K3P0WS2/p1519150596000410