Accessibility: make TrackPanel more accessible using Narrator

Problem:
Using the Narrator screen reader on Windows 10, there are a couple of issues:
1. After making a change to solo/mute/selectedness, the name and state of the track is not automatically read.
2. Text sent to TrackPanelAx::MessageForScreenReader() is not read.

Fixes:
After a focus event, if the focus has not changed, Narrator does not read the name, even if the name has changed. So:
1. In TrackPanelAx::Updated(), add a name change event. (The focus event has been retained to keep Window-Eyes happy until we stop supporting it.)
2. In TrackPanelAx::MessageForScreenReader(), change the focus event to a name change event. Note the Window-Eyes does not read these messages anyway, because when the role is set to wxROLE_NONE it does not read the name. So there's no point in including a focus event for the benefit of Window-Eyes.

src/TrackPanelAx.cpp

@@ -191,12 +191,17 @@ void TrackPanelAx::Updated()
    auto t = GetFocus();
    mTrackName = true;
 
-   // logically, this should be an OBJECT_NAMECHANGE event, but Window eyes 9.1
-   // does not read out the name with this event type, hence use OBJECT_FOCUS.
+   // The object_focus event is only needed by Window-Eyes
+   // and can be removed when we cease to support this screen reader.
    NotifyEvent(wxACC_EVENT_OBJECT_FOCUS,
                mTrackPanel,
                wxOBJID_CLIENT,
                TrackNum(t));
+
+   NotifyEvent(wxACC_EVENT_OBJECT_NAMECHANGE,
+      mTrackPanel,
+      wxOBJID_CLIENT,
+      TrackNum(t));
 #endif
 }
 
@@ -217,7 +222,7 @@ void TrackPanelAx::MessageForScreenReader(const wxString& message)
       mMessageCount++;
 
       mTrackName = false;
-      NotifyEvent(wxACC_EVENT_OBJECT_FOCUS,
+      NotifyEvent(wxACC_EVENT_OBJECT_NAMECHANGE,
                mTrackPanel,
                wxOBJID_CLIENT,
                childId);