feat: Detect that user is using Gradience Flatpak and automatically enable Flatpak theming

[tfuxu]

Edit: Maybe not automatically, but on setup we could ask user if he wants to enable Flatpak theming.

[daudix]

Maybe just make flatpak theming options enabled by default in any case?

[tfuxu]

Not everyone has flatpak framework installed

[daudix]

Ah, makes sense