Rename private flag Action#_processing -> Action#_started.
Rename private flag Action#_processing -> Action#_started.

More accurately reflects meaning of the flag.
orbitjs/orbit.js,SmuliS/orbit.js,jpvanhal/orbit.js,orbitjs/orbit.js,jpvanhal/orbit-core,orbitjs/orbit-core,jpvanhal/orbit.js,jpvanhal/orbit-core,SmuliS/orbit.js