Ability to exit on escape key

[ori-sky]

The engine requires input capabilities and the ability to act on inputs; for example, exiting when the escape key is pressed.

[ori-sky]

This issue can be broken down into steps, each of which could perhaps be opened as its own issue.

• Currently the renderer must deal with all GLFW stuff, so logically-speaking it must also handle input events.
• GLFW input handling is done in the IO monad and therefore we have no access to engine state at this time. A message channel must be implemented to allow various events to be relayed from external sources - such as GLFW's input handler - back into the core of the engine. This should be implemented generically as we want to support potential future external sources/callbacks, as well as the ability to communicate to the main thread from worker threads.
• A system must be devised to store input states and allow lookup by Key in constant time. Ideally, input states should be stored in a generic structure similar to engine Storage, as we don't want to hard-code input structures into the engine state for every future possible source of input. Doing this generically will also, again, allow us to use it for other purposes in the future.