Add support for Flatpak

[louib]

This commit adds support for distributing the application using Flatpak, the Linux distribution-agnostic packaging tool.

This PR is still a work in progress. After this gets merged, I will go through the app submission process to publish a version on Flathub.

TODO

• [x] Allow compiling the app without the tests.
• [x] Figure out why the tips database is not populated in the Flatpak environment.
• [ ] Test the .desktop file and metainfo file with a local install
• [ ] Add flatpak-builder instructions to the dev documentation.