Ensure we have "+" button when adding children to wxTreeCtrl item

The control doesn't update the button itself, even in the recent Windows
versions, so do it ourselves.

It might be better to use I_CHILDRENCALLBACK and handle TVN_GETDISPINFO,
but this looks like a safer change, so do this for now.

See wxWidgets#23718, wxWidgets#23734.

(cherry picked from commit 3ed273c)

Author: vadz

docs/changes.txt

@@ -325,6 +325,7 @@ wxMSW:
 - Fix position of wxStaticBox label after DPI change (Maarten Bent, #23740).
 - Fix background colour of empty cells in wxDataViewCtrl (#23708).
 - Fix possible (even if rare) crash in printer progress dialog (#23927).
+- Fix "+" button not appearing in wxTreeCtrl after adding a child (#23718).
 
 wxOSX:
 

src/msw/treectrl.cpp

@@ -1547,15 +1547,9 @@ wxTreeItemId wxTreeCtrl::DoInsertAfter(const wxTreeItemId& parent,
     tvIns.item.lParam = (LPARAM)param;
     tvIns.item.mask = mask;
 
-    // apparently some Windows versions (2000 and XP are reported to do this)
-    // sometimes don't refresh the tree after adding the first child and so we
-    // need this to make the "[+]" appear
-    //
-    // don't use this hack below for the children of hidden root nor for modern
-    // MSW versions as it would just unnecessarily slow down the item insertion
-    // at best
+    // Without this, the tree doesn't show a "+" button when we add the first
+    // child, at least after removing the children previously (see #23718).
     const bool refreshFirstChild =
-        (wxGetWinVersion() < wxWinVersion_Vista) &&
             !IsHiddenRoot(parent) &&
                 !TreeView_GetChild(GetHwnd(), HITEM(parent));