Move status- and appbar to new component and make statusbar translucent

This moves the status- and appbar to a new component called StandardHeader, which makes it easier to define a custom header for things like the Profile, and components that will use the SearchHeader introduced in !51 (merged). Now the navigator does not contain the statusbar anymore, it is easier to define a custom behaviour for new headers.

Also, the sidebar and profile image in Profile now continue in the statusbar.

I'm not able to test it, but it should still work for iOS, too.