The Extensions dialogue assumed that it had a header bar. Since this is
not always the case, conditional checks were added to prevent
GtkHeaderBar code from running. This also fixes a crash when you
double-click to expand the extension description.
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).
