CLR event support

[jakubmisek]

Allow add/remove callable to the .NET/C# event

for example having C# class X as below:

class X {
  public event e;
}

suggesting to allow two syntax approaches in PeachPie:

• [x] treat event as a fake object with add() and remove() methods: $x->e->add( function() {} ).
• [x] handle += and -= operators: $x-> e += function () {}
• [ ] event will be callable as well (the fake object will have __invoke method)

Note, remove of callables from .NET event might not work (since the instance of the delegate won't be the same), so the on() method should return an instance of a special class with "Dispose" or "Close" methods which would remove the delegate from the event as it's more common in other languages.

[jakubmisek]

add/close implemented in https://github.com/peachpiecompiler/peachpie/commit/232f2921390d99bd513810c381426072afa5f170