MapUpdater doesn't accept AbstractObservables{T} other than Observables{T}

[hhaensel]

Currently, Observables throws an error when trying to map!(any, r, o) where o is a normal Observable and r is of a different observable type R{T} <: AbstractObservable{T}. The root cause is the definition of MapUpdate:

struct MapUpdater{F, T} <: Function
    f::F
    observable::Observable{T}
end

map!(), map() and connect!() all support AbstractObservables.
Probably, the only thing that would need to be adpated is to change Observable{T} to AbstractObservable{T}.

For my use case that worked, but I may be overlooking something.

[hhaensel]

Please have a look for an MWE here.