Generic events API good enough? Plus removal of @onbutton?

[essenciary]

@hhaensel @PGimenez Here's a demo of a server side button click event:
https://github.com/GenieFramework/StippleUI.jl/blob/master/demos/tables/button.jl

I think this by itself is great - and yes, we can add @onclick(:foo) as a wrapper for @on(:click, :foo) but it's of questionable value.

Given that server side events work so simple and nicely, I propose we remove the @onbutton?