OpenXmlPartRootElement should raise the IPartRootEventsFeature.Changed on reload

@twsouthwick, I'm not suggesting a change here but wanted to note that only `Saving` and `Saved` events are raised by the `OpenXmlPartRootElement`. However, I'm thinking we might also want to have `OpenXmlPartRootElement` raise the `Reloading` and `Reloaded` events that you have already defined. For example, consider the case where the `_rootXElement` was deserialized from an `OpenXmlPartRootElement` instance that was changed after loading it from the `OpenXmlPart`. If the user discards those changes by calling the `Reload()` method, the `_rootXElement` should also be discarded and loaded again once it is accessed.

_Originally posted by @ThomasBarnekow in https://github.com/OfficeDev/Open-XML-SDK/pull/1021#discussion_r706880297_