Migrate to WebExtensions

[keithbowes]

Firefox (and presumably SeaMonkey) won't support legacy extensions come the end of the year, so this extension really needs to be migrated to WebExtensions. Hopefully, some code can be salvaged (preferably, both types of extensions can both be available in the same package, depending on browser capabilities). Maybe this will also allow it to work on non-Gecko browsers.

[RoestVrijStaal]

I'd recommend to keep the logic of user-agent-switcher separate from the API-calls of the three SDKs as much as possible.

When switching to WebExtensions-only is the inevitable way to go (not unrealistic since you're doing this in your free time), please put some hints here and there to make it easier for Pale Moon users to fork it to an worthy XUL/XPCOM-addon.