fix: have onDrop handler methods correctly return booleans

Have the onDrop methods, used as handlers for Gtk.DropTarget drop
signals, return booleans as required by Gtk.DropTarget.
https://docs.gtk.org/gtk4/signal.DropTarget.drop.html
This change doesn't seem to have a practical impact, but its good to
follow the API correctly anyway.

src/prefsModules/PrefsBoxOrderItemRow.js

@@ -86,7 +86,7 @@ export default class PrefsBoxOrderItemRow extends Adw.ActionRow {
     onDrop(_target, value, _x, _y) {
         // If `this` got dropped onto itself, do nothing.
         if (value === this) {
-            return;
+            return false;
         }
 
         // Get the GtkListBoxes of `this` and the drop value.
@@ -138,5 +138,7 @@ export default class PrefsBoxOrderItemRow extends Adw.ActionRow {
             valueListBox.saveBoxOrderToSettings();
             valueListBox.determineRowMoveActionEnable();
         }
+
+        return true;
     }
 }

src/prefsModules/PrefsBoxOrderListEmptyPlaceholder.js

@@ -31,5 +31,7 @@ export default class PrefsBoxOrderListEmptyPlaceholder extends Gtk.Box {
         ownListBox.determineRowMoveActionEnable();
         valueListBox.saveBoxOrderToSettings();
         valueListBox.determineRowMoveActionEnable();
+
+        return true;
     }
 }