Add wxRegKey::GetHkey()

Provide access to the underlying HKEY, especially as we already allow
setting it.

include/wx/msw/registry.h

@@ -139,6 +139,8 @@ public:
   bool  IsOpened() const { return m_hKey != nullptr; }
     // for "if ( !key ) wxLogError(...)" kind of expressions
   operator bool()  const { return m_dwLastError == 0; }
+    // return the associated HKEY, possibly null if the key is not opened
+  WXHKEY GetHkey() const { return m_hKey; }
 
   // operations on the key itself
     // explicitly open the key (will be automatically done by all functions

interface/wx/msw/registry.h

@@ -236,6 +236,15 @@ public:
     */
     bool GetFirstValue(wxString& strValueName, long& lIndex);
 
+    /**
+        Return the associated HKEY handle.
+
+        It may be null if the key is not opened.
+
+        @since 3.3.1
+     */
+    WXHKEY GetHkey() const;
+
     /**
         Gets information about the key. Returns @true if successful.