Qt: Start capture from the command line.

Fill in the capture-related mise en place so that -k and -i work. Get
rid of global_capture_session in ui/qt and make it a member of
MainWindow.

Copy over privilege checking from GTK+.

Move the global capture session struct to MainWindow.

Change-Id: Iab5ec683860a40255a7e1d82e3872ced24fd55cb
Reviewed-on: https://code.wireshark.org/review/4382
Reviewed-by: Gerald Combs <gerald@wireshark.org>