Get correct version of an app

Related to #8929

We should get the version of the app. Not of the appfolder. Else there
is no way to properly compare the versions.

Now note that installing in 1 go will still fail. But at least on the
next page load the new version will be properly detected.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>