#2477 undo history is too coarse

[Leo]

Something that undermines a lot the ergonomy of TeXstudio is its undo history. I understand that when the user types in a dozen characters of text, we don't want Ctrl+z to undo just one character, and bundling together a few key strokes is a good idea. However, text typed by the user is being bundled with shortcuts, Ctrl+v, etc etc, and this is really annoying. I believe it would be sensible to have each command or shortcut should be a separate item in the undo history, and only pure text shoud be bundled.