Dumpcap+Qt: Add support for `-a packets:NUM` and `-b packets:NUM`.

Dumpcap+Qt: Add support for `-a packets:NUM` and `-b packets:NUM`.

Add the ability to rotate files after a specified number of packets (`-b
packets:NUM`). Move some condition checks to capture_loop_write_packet_cb.

Add `-a packets:NUM` in order to be consistent. It is functionally
equivalent to the `-c` flag.

Add a corresponding "packets" option to the Capture Interfaces dialog
Output tab.

Add initial tests for autostop and ringbuffer conditions.

Change-Id: I66eb968927ed287deb8edb96db96d7c73526c257
Reviewed-on: https://code.wireshark.org/review/30534
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>