Remove articles/chapters from book

I added a button (similar to add) to allow removing chapters and articles. 

It behaves exactly the same way, with a confirmation dialog prior to deleting.
As mentioned in the confirmation message, this does not take care of the book files, but only about the summary entry.

I needed it personally often, when I found myself quitting the app, changing SUMMARY.md manually and reopening the editor. One does not know the structure of the book directly, so for example I tend to move articles around, leaving empty chapters behind - which I can now delete.

Tested it with a new book, with an existing book, with empty articles, articles with chapters and chapters only - hope I found all test cases...