Ensure the Postback Sequence Counter is updated after potential Javascript altering the site

Currently, it is possible that actions, executed by Javascript (like setting the focus on specific items) interfere with a test, because they happen after the PostbackSequenceCounter was counted up.

We should make sure that the PostbackSequenceCounter is updated last after a postback, to make sure that all actions triggered by the postback have passed and the test can safely continue.