XBMC - Add functionality to show and adjust XBMC volume through an item of type...

... Dimmer.
Since the Volume items will be defined as Dimmer items, they will have to be able to deal with 'string' commands like 'ON', 'OFF',  'INCREASE',  and 'DECREASE', which is why I kept volume as a String type in this method. 

I did change some of the code below, however, to reduce the number of BigDecimal instantiations, as they were rather ugly indeed.