Extension Manager 0.3

The second feature update to extension-manager. Highlights include:

 - New app icon
 - View comments and reviews
 - Support for in-app updates
 - Sort extensions by enabled status

The full list of changes are as follows:

General
 - Add new app icon

Features
 - In-app updates
 - Display comments and reviews
 - Display extension errors to user
 - Handle 'out of date' extensions
 - In-app release notes on first run
 - Allow installing unsupported extensions
 - Option to sort extensions by enabled status
 - Focus search entry on first open

Maintenance
 - Add badges for build, version, license, downloads to readme file
 - Use latest meson features
 - Run appstream validate test without network

Translations
 - Adds context to various translation strings
 - Adds Norwegian Nynorsk
 - Adds Indonesian
 - Adds Occitan
 - Updates Spanish
 - Updates Italian
