Support multi-model pages

[hhaensel]

This PR fixes at least part of what was requested in #171.
It goes together with a respective PR in Genie.

We now support multiple models on a single page, even models of the same type.
Each model has its own websocket connection, with its own keepalive timer and ws_disonnected field.
The reconnection loop issue has also been fixed.

MWE:

using Stipple, Stipple.ReactiveTools
using StippleUI

@app MyApp1 begin
    @in i = 0
    
    @onchange i begin
        @notify("i has changed $i")
    end
end handlers1

@app MyApp2 begin
    @in j = 0

    @onchange j begin
        @notify("j has changed $j")
    end
end handlers2

@app MyApp3 begin
    @in k = 0

    @onchange k begin
        @notify("k has changed $k")
    end
end handlers3

ui(field = :i) = () -> row(card([
    cardsection(h6("Websocket disconnected"), @if(:ws_disconnected))
    cardsection(numberfield(String(field), field))
    cardsection(textfield("$field text", field))
]))

route("/") do
    global model1 = init(MyApp1, channel = Stipple.channelfactory()) |> handlers1
    global model2 = init(MyApp2, channel = Stipple.channelfactory()) |> handlers2
    global model3 = init(MyApp3, channel = Stipple.channelfactory()) |> handlers3
    page([model1, model2, model3], [ui(:i), ui(:j), ui(:k)], pagetemplate = (x...) -> row([x...]), "v-cloak") |> html
end

route("/multi") do
    global models = [init(MyApp1, channel = Stipple.channelfactory()) |> handlers1 for _ in 1:5]    
    page(models, fill(ui(), length(models)), pagetemplate = (x...) -> row([x...]), "v-cloak") |> html
end

up(open_browser = true)