application: add option to specify the theme and json file
application: add option to specify the theme and json file

These files will be used instead of the default files inside the
application gresource.
https://phabricator.endlessm.com/T13538

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib