Split, merge, and delete functionality

[vkbo]

This PR adds the functionality to:

• Permanently delete files from the project. When a file is deleted again from the Trash folder, the user is given the option to permanently remove the file from the project and delete it from disk.
• Merge a folder of documents into a single document. This creates a copy of the content in a folder and merges everything into a single document. The originals can be deleted if the user wants to, but this is not done automatically.
• (ToDo) Split a single file into a folder of files split by header level. The header level, 1-4, to split on can be selected by the user. The original document is not deleted.

This PR completes Issue #63.