Full aspect toggling implementation

- Move aspect.png IO to PathFinder, so all file reading is in the same
place
- Images are now stored with the aspects
- Tweaked the math for alpha masking ImageIcons (now multiplys by
existing alpha, instead of just straight clamping it)
- Aspects now have an "Enabled" state that gets changed via the toggle
buttons
- Added some checks in necessary places to the PathFinder/PathChecker
code to ensure disabled aspects aren't considered for the path, but can
still be valid start/end aspects.