SG-41715: Send internal event on about to show menu (AcademySoftwareFoundation#1065)

### [SG-41715](https://jira.autodesk.com/browse/SG-41715): Send internal
event on about to show menu

### Summarize your change.

Send an internal event on about to show top menu bar. Note that this fix
needs a second PR in the Live Review plugin to fix the issue.

### Describe the reason for the change.

A race condition is happening during Live Review sessions that is
preventing the menu items to stay blocked while RV is handling graph
changes while rebuilding the trop menu bar. To avoid making too much
changes to the code at this moment, I proposed a quick fix where we
simply make sure to inform the menu is about to be shown so that we can
make sure to disable again the required categories.

### Describe what you have tested and on which operating system.
The repro steps in the ticket were successfully tested on macOS.

Signed-off-by: Éloïse Brosseau <eloise.brosseau@autodesk.com>

Author: eloisebrosseau

src/lib/app/RvCommon/RvDocument.cpp

@@ -1469,6 +1469,7 @@ namespace Rv
 
     void RvDocument::aboutToShowMenu()
     {
+        session()->userGenericEvent("about-to-show-menu", "");
         if (QMenu* menu = dynamic_cast<QMenu*>(sender()))
         {
             QList<QAction*> actions = menu->actions();