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/uncommentselection.cpp

@@ -29,6 +29,7 @@
 #include "utils/multitextcursor.h"
 
 #include <QTextBlock>
+#include <QTextCursor>
 #include <QTextDocument>
 
 namespace Utils {