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

interface/wx/bmpbuttn.h

@@ -99,6 +99,24 @@ public:
                 const wxValidator& validator = wxDefaultValidator,
                 const wxString& name = wxButtonNameStr);
 
+    /**
+        Creation function for two-step creation of "Close" button.
+
+        It is usually not necessary to use this function directly as
+        NewCloseButton() is more convenient, but, if required, it can be called
+        on a default-constructed wxBitmapButton object to achieve the same
+        effect.
+
+        @param parent The button parent window, must be non-@NULL.
+        @param winid The identifier for the new button.
+        @param name The name for the new button.
+
+        @since 3.1.5
+     */
+    bool CreateCloseButton(wxWindow* parent,
+                           wxWindowID winid,
+                           const wxString& name = wxString());
+
     /**
         Helper function creating a standard-looking "Close" button.
 
@@ -109,10 +127,13 @@ public:
 
         @param parent The button parent window, must be non-@NULL.
         @param winid The identifier for the new button.
+        @param name The name for the new button (available since wxWidgets 3.1.5)
         @return The new button.
 
         @since 2.9.5
      */
-    static wxBitmapButton* NewCloseButton(wxWindow* parent, wxWindowID winid);
+    static wxBitmapButton* NewCloseButton(wxWindow* parent,
+                                          wxWindowID winid,
+                                          const wxString& name = wxString());
 };