FEAT: Combine getDirtyCount with undo/redo

Currently the changesCount is only increased regardless of undo and redo. I wonder if there is an application where the actual value is used or if this is just a boolean indicator for changes.

If it is just used as boolean, it would be possible to instead add a flag to the undo state and mark the saved position. This way it is possible to signal changes only when the undo stack does not point to the saved position on undo and redo.

Something like this seems to work:

        var um = editor.UndoManager;
        var changepoint = null;
        var hasChange = false;

        function findChange() {
            var s = um.getStack();
            var idx = s.pointer;
            var old = hasChange;
            
            if(idx != -1) {
                hasChange = changepoint !== s.models[idx];
            } else {
                hasChange = changepoint !== null;
            }
            
            if(old != hasChange) {
                console.log('changed', hasChange);
            }
        } 
        
        function saveChange() {
            var s = um.getStack();
            var idx = s.pointer;

            if(idx != -1) {
                changepoint = s.models[idx];
            } else {
                changepoint = null;
            }
        }

        editor.on('undo', findChange);
        editor.on('redo', findChange);
        editor.on('load', function() {
            var i = um.getInstance();
            var add = i.constructor.prototype._addToStack;
            i.constructor.prototype._addToStack = function() {
                var ret = add.apply(this, arguments);
                findChange();
                return ret;
            }
        });