Add "Hold" key press state support

[liamw9534]

Certain operations lend themselves well to using the 'hold' key press state e.g., volume control operations. For certain classes of input device, 'hold' works well e.g., a PS/2 or USB keyboard. But BT speakers tend to perform very poorly and can randomly spit-out many 'hold' events, presumably because of poor hardware design, even when the user did not actually hold down the key.

For now I'm marking this as won't fix.

[piotrkochan]

+1