Port to ESM / GNOME 45 Support

[d-k-bo]

Fixes #145.

Most changes just follow the Official Porting Guide. Additionally, the following changes are necessary:

• Replace Mainloop.* calls with equivalent GLib.* calls
• Replace some global variables with class fields
• Extract settings keys into a separate module so the can be imported by both the extension and the preferences
• Wrap preferences in a Adw.PreferencesPage and Adw.PreferencesGroup

While testing on my main machine (Fedora 39 Beta) using a nested instance of GNOME Shell, I ran into multiple database corruptions, but I couldn't replicate this issue in a GNOME OS VM. I also don't see how this can be caused by these changes.

Since these changes aren't compatible with GNOME <45, it would make sense to create a final release for these versions before merging this PR.

[SUPERCILEX]

Thanks for doing this! I don't have gnome 45 so can't actually test this, but the changes look fine so I'll trust you. :) And yeah, if you have two instances of the extension running at once, chaos will ensue so don't worry about that.

I published the gnome 45 version of the extension which is pending review: https://extensions.gnome.org/review/45415. If stuff's broken once that goes through, let me know and we can work on fixes. If all goes smoothly, thanks again!

[SUPERCILEX]

Alrighty, the extension is live. Can you update and confirm it works?

[d-k-bo]

That was quick, thanks for merging and addressing the review comments! I switched to the extensions.gnome.org version and it works fine.

[SUPERCILEX]

Hell yeah!