Add support for .flatpakref file handling

[brunos3d]

Summary

This PR adds native support for handling .flatpakref files in Warehouse.

Changes:

• Added MIME type registration for .flatpakref files
• Implemented file handling in application
• Updated desktop entry and metainfo
• Added necessary filesystem permissions
• Improved command line handling

Testing:

• Double-click .flatpakref file opens in Warehouse
• Drag and drop .flatpakref works
• Command line opening works (warehouse file.flatpakref)
• "Open With" menu shows Warehouse for .flatpakref files

Fixes/Closes #76

Demonstration

Screencast.From.2025-01-30.16-21-57.mp4

[brunos3d]

Hello, @heliguy4599. If it possible, I would like to ask you to update the app version before merging this PR, idk if it's automatically updated or if manually. Anyways thank you for this amazing tool. I hope my contribution helps it to grow even more.

[heliguy4599]

Wow, thank you so much! Really quickly, can I ask what you mean by "update the app version"? Do you just mean to literally increment the number, or are you asking for something more involved?