VariousPlugins: Fix forward declarations

The pattern is to use QT_[BEGIN/END]_NAMESPACE, instead of
QT_FORWARD_DECLARE_CLASS.

Change-Id: I9da39ee003e1d477a5f56980c218e275706bb3f4
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/todo/settings.h

@@ -6,7 +6,9 @@
 
 #include "keyword.h"
 
-QT_FORWARD_DECLARE_CLASS(QSettings)
+QT_BEGIN_NAMESPACE
+class QSettings;
+QT_END_NAMESPACE
 
 namespace Todo {
 namespace Internal {