Add configuration option to set barcode scanner prefix/postfix.

[scottmc]

From README

Bar Code Scanner:
Your Scanner should be configured in such a way that the ascii char 92 is used as post an prefix. If your scanner don't use this number you have to edit the value in mainwindow.cpp and recompile.

Seems this could be set in a preferences menu, so that end users wouldn't have to recompile anything.