fix: use generic flatpak command in .desktop (fixes #127)

[gabrielalmir]

Addresses #127.

This PR replaces the absolute path /usr/bin/flatpak with a generic flatpak command in the .desktop file, improving compatibility across various Linux distributions (like NixOS).

[gabrielalmir]

I would like to open a discussion regarding the implications of switching from an absolute path (/usr/bin/flatpak) to a generic "flatpak" command in the ".desktop" file.

While this change fixes compatibility for distributions like NixOS, there are potential edge cases to consider:

1. PATH availability in GUI sessions: Desktop environments sometimes have a more restricted PATH than interactive shells. Could this cause the launcher to fail on systems where flatpak is installed in a non-standard location not included in the session's environment?
2. Security & Binary Hijacking: Relying on PATH search order theoretically allows for binary spoofing if a different "flatpak" executable exists earlier in the search path.
3. Cross-Distro Consistency: Is there a consensus or standard preference among major desktop environments (GNOME, KDE, etc.) regarding absolute vs. relative paths in Exec lines for Flatpak applications?

I'd love to hear the community's thoughts on this.

[emsspree]

Cross-Distro Consistency: This is handled differently by every distro. Some prefer comfort (without path), and some prefer security (with path).

As PhotoGIMP should work out of the box for everyone, it should use flatpak without path.