Add GUI display

[RubbaBoy]

Add a GUI that shows the input image, and then bounding boxes around found characters, with the ability to hover over them/click them to see things like potential other matches, thresholds, and relations to other characters, etc. Currently, there is no way to easily do this which makes debugging a pain.

This could make higher-level debugging easier in the future when more fonts are supported and developers are taking advantage of internal APIs. This isn't high-priority, but potentially could be added for v3 of the OCR.

Note: This would not be in the core API of the OCR, but either in the current demo module or another new gui module meant just for GUI handling.