Use wxString rather than QString methods

It's preferable to use code which is simpler to understand to wx
developers not necessarily familiar with Qt API and which can also be
reused with the other ports if necessary.

In this particular case, using wxString::AfterFirst() also results in
shorter and more clear code too.

See https://github.com/wxWidgets/wxWidgets/pull/1544

src/qt/menuitem.cpp

@@ -174,15 +174,11 @@ wxQtAction::wxQtAction( wxMenu *parent, int id, const wxString &text, const wxSt
 void wxQtAction::UpdateShortcutsFromLabel(const wxString& text)
 {
 #if wxUSE_ACCEL
-    QString qlabel = wxQtConvertString( text );
-    int index = qlabel.lastIndexOf( QChar( '\t' ) );
-
-    if ( index != -1 )
+    const wxString accelStr = text.AfterFirst('\t');
+    if ( !accelStr.empty() )
     {
         QList<QKeySequence> shortcuts;
-        QString shortcut_key = qlabel.remove( 0, index+1 );
-
-        shortcuts.append( QKeySequence( shortcut_key ) );
+        shortcuts.append( QKeySequence( wxQtConvertString(accelStr) ) );
         setShortcuts( shortcuts );
     }
 #endif // wxUSE_ACCEL