Gui: Add ability to easily lock Toolbars from UI

This adds the Std_ToggleToolBarLock action mentioned in #4992. It is
exposed in the context menu of toolbar and also in the view -> toolbars
app menu.

src/Gui/Workbench.cpp

@@ -620,6 +620,7 @@ void StdWorkbench::setupContextMenu(const char* recipient, MenuItem* item) const
 
 void StdWorkbench::createMainWindowPopupMenu(MenuItem* item) const
 {
+    *item << "Std_ToggleToolBarLock";
     *item << "Std_DlgCustomize";
 }