Extend handling of app.open message
After opening a UI page, we sometimes send an additional message to it in order to trigger some custom behavior (i.e. open the "add filter list" dialog when clicking on an abp-subscribe link; focusing a section in the desktop settings page). Since those messages originate from either a UI page or a content script, we could extend the "app.open" message to pass such additional data.
Originally reported as trac#3154.
What to change