Add action for releasing state

[jtroo]

Use case:

Let's say I want lalt as a multi-use key that acts as lalt as well as layer-toggle at the same time. Some desired actions in the new layer want lalt to be held down the whole time (1) since repeated press-release of alt results in the wrong behaviour. Other actions (2) actually don't want alt to be pressed at all.

To achieve (1), one could use (multi lalt (layer-toggle other-layer)). To achieve (2), a theoretical action (multi (release lalt) <un-alted key>) could work.