Add 'key on' and 'key off' intents for more flexibility of intents

[sudomain]

Developer TODO (don't remove)

• update documentation

I originally phrased this poorly in #1216 . Rephrasing here for clarity:

Many of my keymaps contain a single "input key code" action without a trigger that I activate using an intent. I would like to be able to create a keymap that holds a modifier key such as ctrl or alt until another intent is received. I propose a new "Hold down" option for key actions that releases the key when the intent with the same UID is received. It would imply enabling "Allow other apps to trigger this key map".