Qt: Add a CaptureFile class.

Wrap the capture_file struct in a QObject which translates cf_cb_* and
capture_cb_* events into signals. Move the global cfile to
capture_file.cpp.

Don't use a void pointer for the capture file struct.

Change-Id: Ic5d5efb4bb1db64aa0247245890e5669b1da723a
Reviewed-on: https://code.wireshark.org/review/5885
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>