Introduce »Updates« navigation section for Apps mgmt instead of confusing notification

[jancborchardt]

Currently there’s still the »You have 3 app updates pending« notification which is not really helpful. We need to move from that to a dedicated left navigation entry for »Updates«.

cc @nickvergessen cause you worked on that before.

[nickvergessen]

Yeah, will have a look after the new app store thing is done