Bring back caching of NotebookEditorInput if required in NotebookEditorInput class

[DonJayamanne]

The caching of NotebookEditorInput has been removed.
This was added previously, probably for diff viewer.

Probably need to review why it was added and bring this back
Removed here #207745

Found that keeping multiple references of NotebookEditorInput was causing a few issues
The current approach is no better, but we fix the bugs

1. If caching is indeed required, then we need to dispose them properly
2. If caching is not required, we still need to dispose the NotebookEditorInput once we're done with this.

The latter can be a debt item for a few of us, once the first is done.

@rebornix /cc