The extension dialog will show details about an extension (long
description, screenshot, upstream URLs, etc.) with this widget.
Update the GimpExtensionList to send a "extension-activated" signal on
double click, and make the extensions dialog react on it to display the
extension details for the activated extension.
This is all still mostly a skeleton GUI, but it is starting to get into
shape.
This is using GTK+3 widgets, so I make sure to keep it well separated
from core code. The gimp-2-10 version will have to rework the GUI, but
the GtkListBox and GtkSwitch are nice and make things easier, so it is
worth using them here).