decompiler: Support sub-templates

Support GtkBuilderListItemFactory syntax by decompiling the nested XML,
rather than preserving it as a string literal.

tests/samples/list_factory_dec.blp

@@ -0,0 +1,11 @@
+using Gtk 4.0;
+
+ListView {
+  factory: BuilderListItemFactory list_item_factory {
+    template ListItem {
+      child: Label {
+        label: bind template.item as <$MyObject>.name;
+      };
+    }
+  };
+}

tests/samples/subscope.blp

@@ -1,8 +1,11 @@
 using Gtk 4.0;
+using Adw 1;
 
 Gtk.BuilderListItemFactory {
   template ListItem {
-    child: Gtk.Label label {};
+    child: Adw.Bin {
+      Gtk.Label label {}
+    };
   }
 }
 

tests/samples/subscope.ui

@@ -11,7 +11,11 @@ corresponding .blp file and regenerate this file with blueprint-compiler.
 <interface>
   <template class="GtkListItem">
     <property name="child">
-      <object class="GtkLabel" id="label"></object>
+      <object class="AdwBin">
+        <child>
+          <object class="GtkLabel" id="label"></object>
+        </child>
+      </object>
     </property>
   </template>
 </interface>]]></property>

tests/samples/subscope_dec.blp

@@ -0,0 +1,12 @@
+using Gtk 4.0;
+using Adw 1;
+
+BuilderListItemFactory {
+  template ListItem {
+    child: Adw.Bin {
+      Label label {}
+    };
+  }
+}
+
+Label label {}