Qt: Add a set of 2x 24-pixel toolbar icons.

Add a script to create each PNG from its source SVG using Inkscape
(which can conveniently be run from the command line).

Change-Id: Ief7410cf76fb1553ce56f1c6bc1ade03ab5db1d6
Reviewed-on: https://code.wireshark.org/review/4964
Reviewed-by: Gerald Combs <gerald@wireshark.org>