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>