Include subtiddler in transclusion recursion detection
Update language flags to generic plugin icon convention
Update author of German translations

Shouldn’t really credit @jermolene, since it is actually @pmario who
has done the work.
Extend control panel plugins tab

Now we display plugin icons, and an accordion containing information
tabs exposed by the plugin.
Temporary icon for core plugin
Add/update plugin readmes

Notice that translators need to translate the core plugin readme
Update plugin documentation