Add possibility to create "Close" bitmap button from XRC

This requires refactoring NewCloseButton() in order to extract
CreateCloseButton() from it, as XRC relies on being able to use two-step
creation which was previously impossible for this kind of buttons.

CreateCloseButton() is rather unusual, as it has to be declared in the
derived, platform-specific class, in order to be able to call its
Create(), but is defined only once in common, platform-independent,
code. However the only alternative seems to be to have a static
function, e.g. InitCloseButton(), which wouldn't be very pretty neither.

Closes https://github.com/wxWidgets/wxWidgets/pull/2118

include/wx/gtk/bmpbuttn.h

@@ -39,6 +39,9 @@ public:
                 const wxValidator& validator = wxDefaultValidator,
                 const wxString& name = wxASCII_STR(wxButtonNameStr));
 
+    bool CreateCloseButton(wxWindow* parent,
+                           wxWindowID winid,
+                           const wxString& name = wxString());
 private:
     wxDECLARE_DYNAMIC_CLASS(wxBitmapButton);
 };