Work around qRegisterMetaType race condition

There appears to be a threading issue in qRegisterMetaType which can
cause it to throw a qWarning about duplicate type converters. This is
caused by a race condition in the Qt code. Still working with them on
tracking down the bug. For now we register the type which is giving us
problems here while we only have the main thread. That should prevent
it from hitting the race condition later on in the code.