This commits add the SH_OS keycode, which works similarly to one shot
layers:
* while pressed, the keyboard is swapped
* if no keys were pressed while it was pressed, the next key press is
swapped
SH_OS also supports chaining with one shot layers:
OSL(x) + SH_OS + key interprets the key press on the oneshot layer.
The ONESHOT_TIMEOUT setting used by one shot keys and layers is also
used by oneshot swap hands. In the above chaining scenario the timeout
of the oneshot layer is reset when swap hands is activated.
Resolves#2682
