Utils: More forward declarations / drop unused headers [T-Z]

Round 1 - focus on headers.
For classes with initial in range [T-Z].

Replace QT_FORWARD_DECLARE_CLASS with QT_BEGIN_NAMESPACE and
QT_END_NAMESPACE and forward declare inside.

Change-Id: I4ac3a8391e6167aa2db3973a9f94a45ac3c8ebd1
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/changeset.h

@@ -30,7 +30,9 @@
 #include <QString>
 #include <QList>
 
-QT_FORWARD_DECLARE_CLASS(QTextCursor)
+QT_BEGIN_NAMESPACE
+class QTextCursor;
+QT_END_NAMESPACE
 
 namespace Utils {