Gui: Remove references to the Addon Manager if no Std_AddonMgr (#23167)

* Gui: Remove references to the Addon Manager if no Std_AddonMgr

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Gui: Switch to disabling controls rather than hiding

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/Gui/Workbench.cpp

@@ -718,8 +718,10 @@ MenuItem* StdWorkbench::setupMenuBar() const
     auto tool = new MenuItem( menuBar );
     tool->setCommand("&Tools");
 #ifdef BUILD_ADDONMGR
-    *tool << "Std_AddonMgr"
-          << "Separator";
+    if (Application::Instance->commandManager().getCommandByName("Std_AddonMgr")) {
+        *tool << "Std_AddonMgr"
+              << "Separator";
+    }
 #endif
     *tool << "Std_Measure"
           << "Std_ClarifySelection"