Qt: Add a set of 2x 16-pixel toolbar icons.

Optimize the toolbar PNGs.

To do: The rest of the PNGs.

Change-Id: Ieed8b0d099655994d43cdc9cf6c9a094d84c1802
Reviewed-on: https://code.wireshark.org/review/4978
Reviewed-by: Gerald Combs <gerald@wireshark.org>