Modify Step Up

## Change
1. Each kill provides a 2-turn buff that gives 1000% movement speed in several moves, and any action other than moving will remove the buff.

## Reasons
1. Removes raw talent level usage & removes old-fashioned callback in Actor.lua
2. Makes Step Up more predictable and reliable, that no matter how much energy you have (which you cannot know in-game), you'll always benefit from Step Up and do not need to guess the remaining steps you could make.
3. Talent level above 5 gives small bonus

See merge request !744