Prevent set_gtk3_theme from crashing

[lavafroth]

Description

When using GSettingsSetting outside a flatpak environment, the added getter, setter and reset methods should prevent the AttributeError when attempting to set the GTK3 theme.

Note: I used python-black for formatting the code and it made a lot of weird opinionated changes.

Fixes #811

Type of change

• [x] Bugfix (Change which fixes an issue)
• [ ] New feature (Change which adds new functionality)
• [ ] Enhancement (Change which slightly improves existing code)
• [ ] Breaking change (This change will introduce incompatibility with existing functionality)

Changelog

• Adds a function to stage the gsettings command to be executed.
• Adds the equivalent setter, getter and reset functions for GSettingsSetting.

Testing

• [x] I have tested my changes and verified that they work as expected

How to test the changes

Run ./local.sh in a non-flatpak environment, apply a theme, making sure to check the GTK3 checkbox. The program should ask you to log out to see the effects.