Rebranding: Change name of Flutter Widgets Explorer to Fluwix

[Lim Chee Kin]

• change package name to com.fluwix. Publish Fluwix App first then phase out Flutter Widget Explorer.
• change URL of Google Play Store for all references such as blog posts, medium articles and documentation
• Change tag line: Fluwix - Flutter Showcases
• Update documentation. "A platform" is better than "An app"

[Lim Chee Kin]

https://github.com/limcheekin/fluwix/tree/rebranding