Effect Processor Improvement

This adds an improvement to both the legacy and new Effect content processor.  Previously if you had and include in your FX file like so:

```
#include "common_stuff.fxh"
```

And you altered the content of the `common_stuff.fxh` file the pipeline would not rebuild any effects.   This was because it doesn't register the dependency during the build.  This was also a problem in stock MS XNA.

This PR has us return the found `#include`s to the processor and register them as build dependencies via `ContentProcessorContext.AddDependency`.  This means when any of these files are changed all referencing FX files are rebuilt as one would expect.

This has been fully tested.