Use template function to avoid code duplication in wxRegConfig

Don't repeat exactly the same code for 3 different types, just use a
template function instead.

Change the existing TryGetValue() helpers to take all output parameters
consistently by pointer, rather than taking only long by pointer and the
other by reference. This allows to isolate the inconsistency of the type
of wxRegKey::QueryValue() parameter in a single place here, hiding it
from the rest of wxRegConfig code.

Add an extra unit test for writing/reading longs to/from wxRegConfig.

include/wx/msw/regconf.h

@@ -88,6 +88,12 @@ protected:
       return self->m_keyLocal;
   }
 
+  // Type-independent implementation of Do{Read,Write}Foo().
+  template <typename T>
+  bool DoReadValue(const wxString& key, T* pValue) const;
+  template <typename T>
+  bool DoWriteValue(const wxString& key, const T& value);
+
   // implement read/write methods
   virtual bool DoReadString(const wxString& key, wxString *pStr) const wxOVERRIDE;
   virtual bool DoReadLong(const wxString& key, long *plResult) const wxOVERRIDE;