Add processing_core crate and webcam support

[tychedelia]

Adds support for webcams and demonstrates a new pattern for non-rendering features.

Webcams are pretty straightforward. They are a new API Object that you can retrieve an Image from, representing the webcam's texture for this frame.

The bigger change here is the introduction of the processing_core crate and moving the primary Bevy App stuff into it. Additionally, we move the actual App initialization code into the main parent processing crate. The idea here is that that top-level crate can manage things like features and which plugins to add. You'll note that webcam is feature flagged. In practice, when we distribute our library, we will enable all features, but it's nice to give users the chance to disable features that they aren't interested in to create a smaller distribution.

Re: #85, we should follow the same pattern.