Refactor mastercook text file check

[cydanil]

Description

This PR refactors the Mastercook text file check: some code was unreachable, and the argument is always a filepath string.
It can therefore all be done in a context manager.

Some dead code from gtk_extras was also removed: is_markup always returns False because an exception is raised.
This is due to an API change in Gtk3, but the check is insignificant enough that it can be removed altogether, rather than fixed.

How Has This Been Tested?

The Mastercook check refactor was tested by adding an integration test.

The is_markup removal was tested by running the application and creating dialogs: editing a recipe and closing it before save, prompting a dialog.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor!

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.