Fix the quality of the app listing on Flathub, by adjusting the app
summary and changing the app icon. This should fix the following
warnings in the "App Listing Quality" report:
- App Icon: Doesn't fill too much or too little of the canvas
- Summary: No weird formatting or punctuation
The Linux version of the app is distributed via Flatpak, this commit
fixes some issues shown in the validation on Flathub. This includes the
summary and branding color and the app icon which should be square
instead of round.
- Adjust "Release" section in contributing guide.
- Disable "X" datasource, since it is not working after the latest API
adjustments.
- Update "version" key and "msix_config.msix_version" key in
"pubspec.yaml" file.
- Change user in "AddSourceReddit" widget help text.
- Enable macOS App Sandbox.
- Adjust logo for Windows version.
