Feature request: add priority filter for notify()

[hhaensel]

I've often come across the need to selectively trigger handlers of observables, e.g. only trigger the first one.
In the past I've worked around that with some weired constructions, but with the introduction of priorities this could be implemented quite easily and in a very clean way.

function Base.notify(@nospecialize(observable::AbstractObservable); priority::Union{Int, Function})
    val = observable[]
    for (p, f) in Observables.listeners(observable)::Vector{Pair{Int, Any}}
        (priority isa Int ? p == priority : priority(p)) || continue
        result = Base.invokelatest(f, val)
        if result isa Consume && result.x
            # stop calling callbacks if event got consumed
            return true
        end
    end
    return false
end

Example

using Observables

o = Observable(1)

on(o) do o
    println("Called with standard priority: $o")
end

on(o, priority = -1) do r
    println("Called with priority -1: $r")
end

notify(o, priority = -1);
# Called with priority -1: 1

notify(o, priority = 0);
# Called with standard priority: 1

notify(o, priority = <(1));
# Called with standard priority: 1
# Called with priority -1: 1

What do you think?