WebUI: Add Web App manifest to allow standalone usage

[lightswitch05]

Why

I love the WebUI and use it regularly on multiple devices. I've installed it on my homepage of my iPhone so it looks like an app of its own - but it just opens in the browser when I click on it. I would like it to function less like another webpage that opens in a browser tab, and more like a app of its own with its own window.

What

To get an app-like feel, all that is needed in a little manifest.json file. So I add that file in this PR. I've tried my best to match the existing logic - adding it to the auto formatter and (hopefully) getting translation working correctly. I don't expect this PR to be approved and merged without some discussion of its merits - but hopefully this gets the ball rolling and we can see where it ends up.

Testing

I'm using a mac... I was able to get a successful run of the build commands, but when I try to actually open the app, it just crashes. I think I might not have everything setup correctly with Qt. So... unfortunately that means this PR is fully untested. If there are any docs I can read, please let me know. Otherwise, maybe this PR will still be useful for a discussion if this feature is even wanted by the project or not. I tried searching in advanced, and I couldn't find any similar requests.

[Chocobo1]

I love the WebUI and use it regularly on multiple devices.

Does the WebUI really work well on non-desktop devices? Do you mind taking a screenshot?

So... unfortunately that means this PR is fully untested. If there are any docs I can read, please let me know.

For development purpose, you can use the "alternative WebUI" function and let it point to your local src/webui/www folder. It should pick up any changes you made in that folder.

[lightswitch05]

I won't say it's an ideal experience, but it works fine on mobile! I would never try messing with configurations, but for simple torrent management functions, it's very convenient. It's easy to load it up, paste in a torrent URL, and go on with my day. Or maybe just load it up and see if that low-seed torrent ever completed.