feat(StatusStackModal): introduce `prevAction` API

This adds a possible `prevAction` hook to `stackLayout.currentItem`
which is triggered (if it exists) when the stack modal's back button is
hit.

ui/StatusQ/src/StatusQ/Popups/StatusStackModal.qml

@@ -60,10 +60,15 @@ StatusModal {
         rotation: 180
         visible: replaceItem || stackLayout.currentIndex > 0
         onClicked: {
-            if (replaceItem)
+            if (replaceItem) {
               replaceItem = null;
-            else
+            } else {
+              let prevAction = stackLayout.currentItem.prevAction
               stackLayout.currentIndex--;
+              if (typeof(prevAction) == "function") {
+                prevAction()
+              }
+            }
         }
     }