Merging all of the XP-Pen code into a single class

[kurikaesu]

So far all of the XP-Pen devices are supported by individual classes that have small differences.
Considering the similarity of it all, we could merge them all into a single class like how the Huion handler is written.

Furthermore it would be possible for the driver to attempt to configure devices that it doesn't know about and still allow the user the ability to use the digitizer & possibly the shortcut keys. Not exactly sure how that would work on the GUI configuration side but I can think of a few ways to do it so that the GUI could still write configuration even if it doesn't really know what the device is.

The benefit of doing it this way is that the driver can then support a wider variety of XP-Pen products which will allow for wider use/acceptance. Users can then submit their device name and device IDs to be included as an "officially supported" device once all the functionality checks out.

For any device that is unique, an issue of course could be cut in order to begin supporting the new unique messages that are being presented.

Thoughts?