Describe the pull request

This PR adds an event infrastructure, which is the foundation for de-coupling certain aspects in apps from their implementation. Currently this only means that Apps define some events, which can be subscribed to. In the future, we might want to add events to other types to allow for a more flexible implementation.

Events are handled similar to those of .NET. Defining an event is straightforward:

Event<EventArgs> myEvent;

Here EventArgs is a default, empty event argument structure, that can be inherited from. Note that all arguments must be passed as members of such a structure and only one structure per event is allowed. The event host may call events using either of the following:

myEvent->invoke(this, { }); // Second argument is EventArgs instance.
myEvent(this, { }); // short-form; analogous to above.

To subscribe to a event, a client can call either Event<>::add or the operator+=:

host.myEvent.add([](const void* sender, EventArgs e) { });
host.myEvent += [](const void* sender, EventArgs e) { };

Event handlers are stored as std::function objects, so using std::bind to regular functions also works. If an event needs to be unsubscribed from, the client can store a token, that identifies the event subscription:

auto token = host.myEvent += [](const void* sender, EventArgs e) { };
host.myEvent -= token; // same as host.myEvent.remove(token);

All samples have been updated to use the the events startup, initializing and resizing. Overloading initialize, run or resize is no longer allowed.