Webapps requiring a content script and a UA override are unpublishable in the click store

For a webapp to have an injected content script, it must have a webapp-properties.json file which lists that script, and --webappModelSearchPath=. on the desktop file Exec line.

It should be possible to override the user agent with a user-agent-override directive in that file too, but it isn't because of bug #1379497.

It *is* possible to add --user-agent-string='something somethingelse' to the .desktop file Exec line, and these two work together fine. However, the click reviewers tools say "should not specify --webappUrlPatterns when using --webappModelSearchPath", which means that *even though this whole combination works*, it is rejected by click review.

This means that it is impossible to publish such an app to the click store, even though it works.