Refactor ActionQueue's internal action properties.
Refactor ActionQueue's internal action properties.

* `content` -> `actions`
* `current` -> `currentAction`

jpvanhal/orbit.js,SmuliS/orbit.js,orbitjs/orbit-core,jpvanhal/orbit-core,jpvanhal/orbit.js,SmuliS/orbit.js,orbitjs/orbit.js,jpvanhal/orbit-core,orbitjs/orbit.js