Add device `Slave Line' for testing purposes

Back-end for Slave Line generated to enable testing of Slave Line
interface.  Is a 1-column device that only does slave control of a single
master channel.  There is no reason to use this for anything but testing,
so it will be removed from the UI (though the code may remain) before
merging to master.
Temporarily add Slave_Line to device manager

For testing purposes only, temporarily include Slave_Line in device
managed.  Will be removed before merge with Master.
Fixed Set Slaves in HWI y<<x instead of x<<y bug

Was binary shifting master channel number by channel state (wrong) instead
of shifting the state over by the appropriate number of places for the
binary channel number.
Remove Slave_Line from Device Manager

Testing is over, so remove Slave_Line from Device manager so it doesn't
confuse end-users.  Effectively reverts 83749a0.