Debugger: Show item count with Qt4's QMap

Contents is still wrong for Clang builds as the
debug information recorded for the node types
is insufficient.

Change-Id: I4c694e5010ade7f3ebbb4a94163d7b44da13c10d
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>