WB-925 Some PNG format icon files cause warnings: The warning messages...

WB-925 Some PNG format icon files cause warnings: The warning messages produced by Qt when reading the PNG images can be safely ignored.  So, the Qt message handler callback was moved out of desktop.cxx and into a new class WorkbenchQtMessageHandler.  This new class also includes support for suppressing messages and currently suppresses the Qt PNG ICC profile messages.  It does not look like wb_command has a Qt message handler, and WorkbenchQtMessageHandler could also be used for wb_command.