[Bug] Microsoft Store - Make it easier to install ffmpeg

[michael-hawker]

Describe the bug
For some reason I didn't have ScreenToGif installed anymore, so I ended up finding it in the Microsoft Store, great!

But I wanted to export as MP4 and hit the need to install ffmpeg, but got this message:

This isn't very friendly, as I don't know where to download it. You should be able to at least point me to where to download it.

Though, I believe you should be able to download and run the application from an app protected area for your app even when distributed within the store (as long as that executable also doesn't try and reach out of that folder). You just can't write or install to an arbitrary place or run an installer.

To Reproduce
Steps to reproduce the behavior:

1. Install app from store
2. Open options->extras
3. Try and download ffmpeg
4. See error

Expected behavior
Able to download/use ffmpeg as non-ms store edition, or at least open browser to location to download manually.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. Windows 10]
• Version [e.g. 2.35.2]

Additional context

Related #684

[NickeManarin]

Hi, when using the Store version, I'll open the browser with a link to download FFmpeg (or the user can download any v4 or v5 version and add the reference manually).