Added "only-watched-property" attribute to directive

Added possibility to set an `only-watched-property` attribute to the fullscreen directive, as going to fullscreen is not always desired when clicking on the element itself. Instead, *only* the watched property should trigger the fullscreen functionality.

Example:

```html
<div fullscreen="isFullscreen" only-watched-property>Lorem ipsum...</div>
```
In this case, only the `isFullscreen` toggles the fullscreen, and not a click on the `div`.