Following {T254478}, the TemplateData dialog now has a maps panel, with a multiline text input widget containing the maps object as a JSON string. The text input widget is disabled.
This task is for enabling the text input widget and making the JSON editable. (Changing the format to something simpler than JSON will be done in a future task.)
Acceptance criteria:
* The multiline text input is enabled
* As the user types, the "Done" action becomes disabled is the JSON is invalid
* As the user types, the "Done" action becomes enabled if the JSON is valid
* On clicking the "Done" action, then the "Apply" action, the changes appear in the wikitext area when the dialog closes
* On clicking the "Back" action, the model will revert to the state before the maps panel was opened, any changes will be discarded