Correct form of mnemonics returned by wxGTK wxMenu::GetTitle().

wxMenu::GetTitle() returned a string in GTK+ format (i.e. using underscores
instead of ampersands) instead of the expected wx one.

This is, of course, the right thing to do and it also fixes
wxMenuBar::FindMenuItem() as a side effect.

Closes #12672.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66175 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk1/menu.h

@@ -73,6 +73,9 @@ public:
     virtual wxMenuItem* DoInsert(size_t pos, wxMenuItem *item);
     virtual wxMenuItem* DoRemove(wxMenuItem *item);
 
+    // Returns the title, with mnemonics translated to wx format
+    const wxString GetTitle() const;
+
     // TODO: virtual void SetTitle(const wxString& title);
 
     // implementation