StyleManager events fire multiple times per change

On a fresh install of GrapesJS, within the demo page (within index.html), add an event listener like so:

`editor.on('component:styleUpdate:width', (model) => {
   console.log(model);
});`

### Issue:
When changing the 'width' value of any element, the 'model' variable will be printed multiple times.

The issue seems to be more severe when listening for 'list' properties such as Font Family, Font Weight or Display. E.g:
`editor.on('component:styleUpdate:font-family', (model) => {
   console.log(model);
});`

For these kinds of properties, I have noticed that the events fire multiple times when the editor is first loaded (before any user interaction), and also multiple times when their value is changed.

### Cause:
After some investigation, i thought the issue may relate to the value of the properties being set within the GrapesJS framework multiple times to accommodate the Undo Manager [(Property.js:58)](https://github.com/artf/grapesjs/blob/dev/src/style_manager/model/Property.js#L58), although I am still unable to determine the underlying cause or provide a fix for the issue myself.

### Symptoms:
Due to this, it will be hard for developers to use the StyleManager events as intended in some cases. For example, if you would like to listen for what the value was changed from via the ._previousAttributes property you will have difficulty, as the event is fired multiple times: and each time the previous attributes specified are different and don't relate to what the user actually chose.