organize-dialog: disable "New notebook" button if entry empty

As described in #181, the "New notebook" button in the Notebooks section of the menu is currently enabled when the name entry is empty, and confusingly does nothing if pressed in this situation.

This MR disables the "New notebook" button by default, and adds a callback that listens to the notify::text signal and enables the button only when there is text in the name entry. This disables the "New notebook" button when the entry is blank, as expected.

Closes #181