Add global hotkey support

[bensmithuk]

Allow devices to be toggled via a hot key so I can use a keyboard shortcut (or macros or streamdeck etc) to turn a device on / off.

[andrew-codechimp]

Nice idea and something I'll definitely see about adding. I'm thinking some sort of list where you add entities like light.office and the usual hotkey picker. The only issue I'll face is my experience with the UI components which I've deliberately kept simple but up for a challenge.