RegistryKey: Move member variable initialization to the ctor-init-list

In cause of doubt this is more efficient, see

https://isocpp.org/wiki/faq/ctors#init-lists

This also fixes the initialization order to match the declarartion order
of the variables in the class which some versions of GCC otherwise would
complain about.

Change-Id: I642f7156d624c2c65f2f3525d813f5289c092f96
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qsettings_win.cpp

@@ -319,11 +319,11 @@ private:
 };
 
 RegistryKey::RegistryKey(HKEY parent_handle, const QString &key, bool read_only)
+    : m_parent_handle(parent_handle),
+      m_handle(0),
+      m_key(key),
+      m_read_only(read_only)
 {
-    m_parent_handle = parent_handle;
-    m_handle = 0;
-    m_read_only = read_only;
-    m_key = key;
 }
 
 QString RegistryKey::key() const