Use GUIDs in save files instead of widget names

[PeterJohnson]

#132 was closed because renaming a widget breaks save file loading. This seems non-ideal, as it means we are forever stuck with whatever initial name was chosen for a particular widget. Instead of using the widget name directly, it would be better to separate the user-visible name from the unique identifier used in save files. One potential way to do this would be for each widget to have a GUID in addition to the user-visible name. The GUID would be used in file save and loading and the user-visible name would be used for display. The other advantage of this approach is that it eliminates the potential for widget name conflicts between different shuffleboard plugins.