Allow choosing the destination for exported GIF

[clearlysid]

Currently when you export a GIF, it gets saved to the user's desktop folder. For someone recording many GIFs over time, this can become quite annoying and will clutter their desktop.

Proposal:

1. When user clicks the Export button after recording, let's open a dialog box allowing them to choose their preferred location instead of defaulting to the desktop all the time.
2. We can use the OS's default dialog box using Tauri's dialog plugin either from the JS side or the Rust side (though I would prefer Rust) to achieve this.

Export button:

I'm imagining a dialog box like this on macOS

and this one on Windows