Add SeaMonkey support to WebExtension

[kyoshino]

From #245 (comment)

I've got the following error while testing the new WebExtension in SeaMonkey.

There was an error during installation: Add-on {8620c15f-30dc-4dba-a131-7c5d20cf4a29} is not compatible with application version. add-on minVersion: 56.0.

If I remove the applications key, the extension can be temporarily installed via about:debugging but can't find toolbar button nor context menu. I guess SeaMonkey doesn't fully support WebExtensions yet, but I'm filing this issue as a reminder.

[MikkCZ]

Do all the features work without the applications in the manifest file? I thought it's necessary to access the list of installed extensions at least. But if I am wrong and the applications is not needed, I would remove it and than just wait for SeaMonkey to support WebExtensions correctly (IIRC the latest version is based on Firefox 51 core, but the API support may be even more behind).

[whimboo]

@kyoshino has the application in the manifest to be named gecko or is that free? It's a hash, so I would assume we could add other applications too.