Support for cross-browser web extensions

[kalikiana]

Aside from its own native extension (plugin) interface Midori should support web extensions. The use cases are:

• Use existing extensions for other browsers
• Facilitate feature development based on web technologies
• Share extensions between platforms (Android, GTK+)

Issues or merge requests should be linked next to the items, which should be checked as they're implemented.

• Recognize manifest.json analogous to *.plugin Initial support for cross-browser Web Extension API #250
• Content scripts/ styles Initial support for cross-browser Web Extension API #250
• Background page/ scripts Initial support for cross-browser Web Extension API #250
• API exposed to content scripts
• Browser action in toolbar Initial support for cross-browser Web Extension API #250
• Page action in urlbar
• Sidebar support Support for sidebar action WebExtension API #297
• Context menu
• Proxy override
• Interception of network requests
• Hide buttons easily via right-click
• Keyboard shortcuts (commands)
• Manage web extensions in the preferences dialog Refactor web extension class for re-use #379
• Localization
• Options accessible from the preferences dialog
• Permissions
• Load extensions from local folders Initial support for cross-browser Web Extension API #250
• Open ZIP archives on the fly Load extensions from ZIP archives #255
• Install extensions from the web
• Side-load extensions locally
• Unit tests
• Theme extensions

See ANDROID-10 for the Android epic

For reference:

• Browser Extensions Spec
• Compatibility with Chrome in Microsoft Edge
• Chrome extensions

[kalikiana]

With initial (built-in) extension having landed, we can look into more specific aspects and interfaces to support as well as UX tasks as outlined in the proposal.