Remember DisplayDevice original resolutions

When refreshing the AvailableDevices list, it is important to set the
original resolution on any DisplayDevices that were previously available
to allow the RestoreResolution() method to work correctly.
Change original_resolution field to a property

As the original_resolution field needs to be accessed from outside the
module, it should be an internal property as opposed to an internal
field
Merge remote-tracking branch 'cwassall/develop' into develop