Review beginner tutorial for Python

Reviewed the Beginner Tutorial Python code.

I used GNOME Builder 43.

While reviewing I assumed the tutorial is performed front-to-back.

• Removed GSettings setup stuff from dark_mode page, since that was already done a few pages before
• Cleaned up open_file_complete on toaster page, to be similar to what we've built in opening_files
• Fixed a bunch of emphasized lines
• Few textual changes
• Examples have been updated to reflect the GNOME Builder 43 project template

Some things to improve still:

• self._native is not needed for dialogs, I think
• No GSettings files have been included in the Python project template
• The Shortcuts UI file is not present in the Python project template
• the display name code is defined in both the load and save code. It's probably better to extract that in its own method.

GNOME Builder bug:

• When I name the project text-viewer the main class is name Text-viewerApplication.