Add plumbing for stat commands ("-z ...").

Trigger dialog creation by passing a method name to
QMetaObject::invokeMethod. I'm not entirely sure this is sane but it
seems to work OK. Move getopt processing further down in the main initialization sequence
to more closely match GTK+ and allow for stat command registration.

Change-Id: I5cd5375fa71dbadac69d528b2ba3bb13598dc3f6
Reviewed-on: https://code.wireshark.org/review/2964
Reviewed-by: Gerald Combs <gerald@wireshark.org>