Create preferences window

[tfuxu]

This PR adds a preferences window with options for setting overrides in Flatpak aplications.
These changes have, to some extent, features requested in issue #276.

Changes

• Create new .blp file for preferences window,
• Make self.settings object accessible to other files,
• Create new item in main-menu menu: Preferences

I'm setting this PR as a draft for now, to check if everything works as expected

Ps. This time, I didn't forget to update POTFILES :P

[0xMRTT]

Also, the ability to configure Flatpak will' go in the welcome dialog too.

[tfuxu]

Also, the ability to configure Flatpak will' go in the welcome dialog too.

Yes, I saw that you started working on it. I could maybe move functions from preferences.py as a module, just so there won't be two implementations of the same feature in different parts of code.

[0xMRTT]

Yes 👍

[tfuxu]

Okay, should be ready for review. I've also added support for gtk3 in functions, so the same functions can be used without need for creating additional changes.