Picker widget show_widget_id generation

The fix for bug 777766 breaks existing Windmill tests and also makes new tests extremely hard to write because the show_widget_id attribute is no longer discoverable from the field name. The fix uses a base63 encoding to convert the field to non-human readable text.

The point of the fix was to remove invalid characters from the field name so that a valid HTML node id could be generated from the field name. This needs to be done another way so that developers can still relate the generated show_widget_id value back to the associated field name. Otherwise windmill tests which rely on this known mapping break.