Remove support for browser plugins
All browser vendors agree: plugins were a mistake. I no longer intend to support browser plugins in Epiphany. Windowed plugins only work in X11, the amount of crash reports from plugins is absurd, it's time for the GNOME Shell browser plugin to go, Evince browser plugin is far too buggy, the only plugin that still really matters is Flash, and Firefox and Chrome are slowly killing that too. If you want Flash, go use one of those browsers or MiniBrowser or whatever. Note that plugins are unrelated to extensions. We do wish to support Firefox/Chrome WebExtensions. Everyone likes extensions.