Add close button to Studio plugin windows.

Plugin windows could only be closed from the Plugins menu. Pass the
plugin's active flag to ImGui::Begin so the window gets a close button
that writes the same persisted state as the menu toggle. Skip the
update callback while the window is collapsed.

PiperOrigin-RevId: 956795070
Change-Id: I61cb5c26e7af935b6bb296f1b89142a72b0daca1

src/experimental/studio/app.cc

@@ -1167,8 +1167,9 @@ void App::BuildGui() {
       ImGui::EndMainMenuBar();
     }
     if (plugin->active) {
-      ImGui::Begin(plugin->name);
-      plugin->update(plugin);
+      if (ImGui::Begin(plugin->name, &plugin->active)) {
+        plugin->update(plugin);
+      }
       ImGui::End();
     }
   });