Do not dynamic_cast QObject, use qobject_cast, it's faster.

qobject_cast doesn't use RTTI to figure out the type, it uses
an internally stored relationship, wich is much faster than
RTTI. I grepped the code for dynamic_cast and there are lots
besides this one, so I'll later try to figure out wich ones
are QObject inheritances and change acordingly.
Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>

src/ui/MainWindow.cc

@@ -399,7 +399,7 @@ void MainWindow::_hideAllDockWidgets(void)
 
 void MainWindow::_showDockWidgetAction(bool show)
 {
-    QAction* action = dynamic_cast<QAction*>(QObject::sender());
+    QAction* action = qobject_cast<QAction*>(QObject::sender());
     Q_ASSERT(action);
     _showDockWidget(action->text(), show);
 }