[application] Use 'url' manifest key for launching "dummy" applications

Use 'url' manifest key for launching "dummy" applications (applications that were created internally
for rendering and arbitrary URL) instead of currently used 'app.launch.web_url'.

Spec: http://w3c.github.io/manifest/#url-member
Merge pull request #1508 from pozdnyakov/ChangeManifestKeyToURL

[application] Use 'url' manifest key for launching "dummy" applications