Ability to invoke "Decode As..." as command-line argument for GUIshark.

Ability to invoke "Decode As..." as command-line argument for GUIshark.

TShark has had the functionality for awhile. While the GUI version
still has ways to change and persist Decode As functionality, adding
command line functionality gives the Decode As from initial launch
of the GUI.
Was also an excuse to refactor a bunch of code out of tshark.c

Bug: 5143
Change-Id: Ie67007d75e897bc06cc9afd9b84372a96b93778c
Reviewed-on: https://code.wireshark.org/review/16008
Reviewed-by: Michael Mann <mmann78@netscape.net>