poll mouse at 20Hz

required until there's a dynamic poll rate, because 10Hz is too
low for magnifiers (lag), mousemark (paints too edgy), mouseclick
(often fails because it's easy to cycle the button state within 100ms)
- and the bouncing icon lags behind the mouse as well ;-)

REVIEW: 111909

cursor.cpp

@@ -166,7 +166,7 @@ X11Cursor::X11Cursor(QObject *parent)
     m_resetTimeStampTimer->setSingleShot(true);
     connect(m_resetTimeStampTimer, SIGNAL(timeout()), SLOT(resetTimeStamp()));
     // TODO: How often do we really need to poll?
-    m_mousePollingTimer->setInterval(100);
+    m_mousePollingTimer->setInterval(50);
     connect(m_mousePollingTimer, SIGNAL(timeout()), SLOT(mousePolled()));
 }