Show number of pages in the corner of the Notebook button#577

The Notebook kind of gets under-used in TTS in general. It could use some love in terms of functionalities, but it would be nice if people even KNEW that it was worth checking out in a mod. (You can write about it in the Notes, but a lot of people even miss that.)

To solve this last bit, I suggest that there be a number in the corner of the Notebook button, similar to a notifications “badge” on an app (but not in red like in apps). This number would just show the number of pages WITH TEXT that are featured in the Notebook. And if that number is 0, the corner number wouldn’t appear.