Use default entry point to start application if not specified

If the --app is not given, we check the installed desktop files
and use the main one if that makes sense.

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5326