Qt: Use wsiconcap while we're capturing.

Set the main window icon to the "wsiconcap" version while we're capturing
similar to the GTK+ version. Verified on Windows. Not sure if this will
do anything on other platforms.

Change-Id: I9b082601a2c47e5f52cc38ac8d9b4d9f5fb9a4d9
Reviewed-on: https://code.wireshark.org/review/4230
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>