Use adapter object to expose tab API to extensions

[The-Compiler]

Originally, I planned to expose the entire tab API to future extensions. However, during an API review it turned out that this probably isn't a good idea - the tab API has some stuff in it which is very specific and should probably only be exposed to the core.

Instead, there should probably be another adapter object in front - then we can only selectively expose the API which seems stable enough to actually make it public.

[The-Compiler]

@jgkamat mentioned a nice idea in IRC: Using decorators in the existing tab API to decide what to expose to the extension API. However, the big drawback of that (compared to hand-written classes) is that it will likely confuse anything doing static analysis or type checking...