Add an environment variable with a list of devices to ignore for tablet mode
The tablet mode code checks whether we are currently in tablet mode based on whether or not we have a touch screen and a pointer. For some devices, a touch screen or pointer device may be detected but we would like to ignore it, since it incorrectly influences the tablet mode.
This adds an environment variable that can be set to a list of devices to ignore when trying to determine whether we are in tablet mode. These devices will be completely ignored when trying to determine if we are in tablet mode.