Refactor: Reuse He.ViewTitle instance to update appbar titles

- Introduced a single `He.ViewTitle` instance for updating the appbar titles.
- Simplified the code by removing redundant instantiations of `He.ViewTitle`.
- Updated the title labels directly on the existing `He.ViewTitle` instance.

src/MainWindow.vala

@@ -128,23 +128,23 @@ public class Mondai.MainWindow : He.ApplicationWindow {
         actions.add_action_entries (ACTION_ENTRIES, this);
         insert_action_group ("win", actions);
 
+        var title = new He.ViewTitle ();
+
         stack.visible_child_name = "select";
-        appbar.viewtitle_widget = null;
-        appbar.viewtitle_widget = new He.ViewTitle () { label = _("Select Component") };
+        appbar.viewtitle_widget = title;
+        title.label = _("Select Component");
         appbar.show_back = false;
         appbar.scroller = s;
 
         to_describe.clicked.connect (() => {
             stack.visible_child_name = "describe";
-            appbar.viewtitle_widget = null;
-            appbar.viewtitle_widget = new He.ViewTitle () { label = _("Describe Issue") };
+            title.label = _("Describe Issue");
             appbar.scroller = s1;
             appbar.show_back = true;
 
             appbar.back_button.clicked.connect (() => {
                 stack.visible_child_name = "select";
-                appbar.viewtitle_widget = null;
-                appbar.viewtitle_widget = new He.ViewTitle () { label = _("Select Component") };
+                title.label = _("Select Component");
                 appbar.show_back = false;
                 appbar.scroller = s;
             });
@@ -153,14 +153,12 @@ public class Mondai.MainWindow : He.ApplicationWindow {
         submit.clicked.connect (() => {
             submit_report.begin (() => {
                 stack.visible_child_name = "submitted";
-                appbar.viewtitle_widget = null;
-                appbar.viewtitle_widget = new He.ViewTitle () { label = _("Report submitted!") };
+                title.label = _("Report submitted!");
                 appbar.scroller = s2;
 
                 appbar.back_button.clicked.connect (() => {
                     stack.visible_child_name = "describe";
-                    appbar.viewtitle_widget = null;
-                    appbar.viewtitle_widget = new He.ViewTitle () { label = _("Select Component") };
+                    title.label = _("Select Component");
                     appbar.scroller = s1;
                 });
             });