#59 Action-based undo/redo

There are many benefits to doing this:

• less confusing code as actions are easy to distinguish
• less chance of memory bugs as it only saves the data that needs to be changed opposed to the entire buffer
• better stability as I've had a heck of a time with the current undo
• easier debugging, actions are easier to trace
• fixes duplicated data bug (very common with the current method)
• similar to other editors (both OpenBSD's mg and Adam Saponara's mle editors use this method
• faster, it's a lot quicker to copy <100 chars than thousands of chars
• action packed because it use actions

Closed with the action-based-undo branch. Merged in to master.