Try to make tap operations in the Qt UI more responsive.

Add single-shot timers to ramp up the tap update interval and update
taps when we finish reading the capture file.

Change-Id: Ia1694b47ffd2705b6a06aa50c21e675a64aefeac
Reviewed-on: https://code.wireshark.org/review/3099
Reviewed-by: Gerald Combs <gerald@wireshark.org>