Qt: Flatten group boxes.

As discussed in change 5185, the "big 3" HIGs discourage the use of
traditional group boxes. Flatten our existing ones.

Change-Id: I68ef4aa9fd0f69a42750bf735df2250a468bfb2d
Reviewed-on: https://code.wireshark.org/review/5263
Reviewed-by: Gerald Combs <gerald@wireshark.org>