Support for Opera Browser

[rthaut]

The Opera web browser also supports Extensions, so it should be possible to bring DeviantArt Filter to Opera now.

Unfortunately, it looks like Opera uses callbacks instead of promises, so supporting Opera may require using Mozilla's Web Extension polyfill, but Opera appears to use the chrome.* namespace, so hopefully the polyfill will just work as-is.

Additionally, there are a few differences in/for the supported Extension APIs between Opera and Chrome. Most notably, it looks like Opera only supports local storage, which means the settings (which use sync storage) will need some additional work to detect storage support and use the best available system.

[bob12111]

We definitely need this for Opera Browser!