Fix autosizing empty columns in wxMSW wxListCtrl

Autosizing the column by double clicking on its separator line reset it
to the default width when the control was empty, which isn't especially
useful.

Set the column size to fit its header instead.

Closes #9926.

See https://github.com/wxWidgets/wxWidgets/pull/1573

src/msw/listctrl.cpp

@@ -2290,6 +2290,21 @@ bool wxListCtrl::MSWOnNotify(int idCtrl, WXLPARAM lParam, WXLPARAM *result)
                 event.m_col = nmHDR->iItem;
                 break;
 
+            case HDN_DIVIDERDBLCLICK:
+            {
+                NMHEADER *pHeader = (NMHEADER *) lParam;
+                // If the control is non-empty, the native control will
+                // autosize the column to its contents, however if it is empty,
+                // it just resets it to some default width, while we want to
+                // still autosize it in this case, to fit its label width.
+                if ( IsEmpty() )
+                {
+                    SetColumnWidth(pHeader->iItem, wxLIST_AUTOSIZE_USEHEADER);
+                    return true;
+                }
+            }
+            break;
+
             case NM_RCLICK:
                 {
                     POINT ptClick;