Add ability to follow UDP stream by index

-z "follow,udp" tshark cli command now supports a stream index
It is now possible to select the UDP stream displayed in Qt GUI (like for TCP)

Change-Id: Ia367f36ea4f60db0fddb997a7e0903c09e172f2d
Reviewed-on: https://code.wireshark.org/review/6083
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>