Use wxGtkValue RAII wrapper in wxGTK code

wxGtkValue class was added back in 3f84cb17ca (Add wxActivityIndicator
control., 2015-03-06), but somehow never used. Start using it now
(better late than never...) as it makes the code simpler, shorter and
more robust.

No real changes.

include/wx/gtk/private/value.h

@@ -17,6 +17,11 @@
 class wxGtkValue
 {
 public:
+    explicit wxGtkValue()
+        : m_val(G_VALUE_INIT)
+    {
+    }
+
     // Initialize the value of the specified type.
     explicit wxGtkValue(GType gtype)
         : m_val(G_VALUE_INIT)