Override Ctrl-A accelerator when wxTextCtrl has focus in wxMSW

This key combination is used for selecting all text, while it's also
relatively common to use it as an accelerator for some menu item.

Resolve the conflict in favour of wxTextCtrl, i.e. let it have this key
when it has focus, while still allowing to use it as an accelerator
otherwise.

src/msw/textctrl.cpp

@@ -2048,6 +2048,7 @@ bool wxTextCtrl::MSWShouldPreProcessMessage(WXMSG* msg)
                     {
                         switch ( vkey )
                         {
+                            case 'A':
                             case 'C':
                             case 'V':
                             case 'X':