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

for more information, see https://pre-commit.ci

* Gui: Switch to disabling controls rather than hiding

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

src/Gui/PreferencePages/DlgSettingsGeneral.cpp

@@ -127,6 +127,8 @@ DlgSettingsGeneral::DlgSettingsGeneral( QWidget* parent )
     const auto visible = UnitsApi::isMultiUnitLength();
     ui->comboBox_FracInch->setVisible(visible);
     ui->fractionalInchLabel->setVisible(visible);
+    ui->moreThemesLabel->setEnabled(
+        Application::Instance->commandManager().getCommandByName("Std_AddonMgr") != nullptr);
 }
 
 /**