Add file extension when saving text files w/ Notebook

[ace-dent]

Describe the bug / feature Saving files with Notebook, should add .txt if the file extension is missing.

Forum post:

When using the Notebook app and saving a file, you have to make sure it ends with .txt or else it won't recognize the file type. -- holipop

To Reproduce Steps to reproduce the behavior:

1. Create a new text document with Notebook.p64.
2. Save the file and enter a filename without extension, e.g. foo.
3. The new file lacks any file extension and cannot be automatically opened with Notebook.

Expected behavior When filenames lack the required file extension, add it.

Software (please complete the following information):

• OS: macOS
• Version: v0.1.0b

[ace-dent]

Fixed! Tested with release v0.1.0d on macOS. Now adds missing .txt extension. 🎉