All files in a window have a common history

This might be against your design goals for this plugin, but for the off chance that you might be ok with it, I am submitting this pull request.

My major changes are as follows -

- All files share a common changelist, so that it is possible to trace back among all of them the history of your modifications.
- The global list dict, now holds different lists for different windows
- I use "points" instead of keys and selections. Points are just an int which is the offset of the cursor, a bit simpler, and easier to understand for me. I still keep most of the key/sel logic to piggy back off of that code
- Key bindings I have changed to be a bit more natural ( <  / > for back / front) but this might be again, against your design
- File open is considered a change.

This is kinda rough tho. It works for the most part but there might be minor bugs. I would rather clean up the key/sel stuff. And the json file stuff is probably broken, because I am not sure I understood what they were for. And my keybinding changes have not affected the vintage mode bindings etc.. I can clean this up a bit more if you are interested in merging.