WIP: make the add-on list a widget

The purpose of this change is ability to create more dialogs which present
a list of add-ons, such as a GUI2 version of the "Install Dependencies"
dialog.

This change is still highly incomplete. First of all, I disabled inline
install buttons and ability to sort the add-on list. In addition, a lot of
functionality that belongs in the add-on list widget is still in the GUI2
add-on manager instead. It is also notable that the widget relies on
several linked groups being defined in the window.

I plan to finish the transition to the add-on list widget, and address the
problems in the previous paragraph, in the future. The linked group issue
is going to require a lot of work in particular: I plan to either make
linked groups grid-specific instead of window-specific (like they are now)
or allow widgets to inject linked groups to the containing window. Either
way, I'm going to change the operation of GUI2 fairly deeply.

data/gui/schema.cfg

@@ -593,6 +593,21 @@
             name="gui"
             min="0"
             max="1"
+            [tag]
+                name="addon_list_definition"
+                min="0"
+                max="-1"
+                super="generic/widget_definition"
+                [tag]
+                    name="resolution"
+                    min="0"
+                    max="-1"
+                    super="generic/widget_definition/resolution"
+                    [link]
+                        name="gui/window/resolution/grid"
+                    [/link]
+                [/tag]
+            [/tag]
             [tag]
                 name="button_definition"
                 min="0"
@@ -1451,6 +1466,22 @@
                                 name="column"
                                 min="0"
                                 max="-1"
+                                [tag]
+                                    name="addon_list"
+                                    min="0"
+                                    max="-1"
+                                    super="generic/widget_instance"
+                                    [key]
+                                        name="install_status_visibility"
+                                        type="string"
+                                        default="visible"
+                                    [/key]
+                                    [key]
+                                        name="install_buttons_visibility"
+                                        type="string"
+                                        default="invisible"
+                                    [/key]
+                                [/tag]
                                 [tag]
                                     name="button"
                                     min="0"