Implement a UI to open projects/datasets via native OS file picker on windows/linux/macos

[nirvn]

Since having merged the WebDAV folder import/download/upload functionality has been merged, QField's own QML local data file picker is (finally) enabled across all platforms. This brings in a few nice positives, including:

• Overall, that just harmonizes behavior across all platforms;
• Any documentation covering local data file picker is now valid across all supported platforms (massive win IMHO);
• On desktop platforms, importing datasets/projects from URL was just not accessible, which was a bummer when users had used this on their mobile devices prior to loading QField on their desktop (smaller win, but not insignificant :) ).

The one thing we had temporarily lost there was the ability to us the OS file picker to open projects/datasets, which was a nice collateral benefit of not having the local data file picker enabled 😉 . This PR re-implements that on desktop platforms through the local data picker screen via a new button located at the bottom right corner of the screen:

[qfield-fairy]

🎉 Ta-daaa, freshly created APKs are available for eb0a68d: arm64-android

Other architectures

• arm-neon-android
• x64-android
• x86-android
• all access arm64-android
• all access arm-neon-android
• all access x64-android
• all access x86-android

[nirvn]

(Cleaning up as we're ending towards the end of this development cycle :) )