Allow opening wxSearchCtrl menu from keyboard with Alt-Down

The search menu could only be opened by clicking the button showing it,
which doesn't accept focus from the keyboard on purpose, so keyboard and
screen reader users couldn't use it at all.

Handle Alt-Down in the text part of the control and show the menu from
there, as suggested in the comments of the original ticket.

Fixes #18457.

Closes #27093.

include/wx/generic/srchctlg.h

@@ -169,6 +169,7 @@ protected:
 
 private:
     friend class wxSearchButton;
+    friend class wxSearchTextCtrl;
 
 #if defined(__WXMSW__) && !defined(__WXUNIVERSAL__)
     // Implement wxMSW-specific pure virtual function by forwarding it to the