Fix checkers piece moveset

[MGPAlpha]

Fixed two issues with the checkers' piece's take ability that I encountered when playing the game at the recent VGDev showcase.

1. Previously it would allow you to target any space with any kind of obstruction, including empty gaps and your own pieces. To force it to only target enemy pieces, I introduced a new remove_unobstructed_by_enemy ActionTrait that keeps only spaces that are obstructed by an enemy piece. I also added a less strict version remove_unobstructed_by_piece that doesn't care what team the enemy is on, as well as new ?obstructedbypiece and ?obstructedbyenemy actions to match the existing ?obstructed.
2. The checkers' take ability didn't actually move the piece. This was fixed by simply editing the grid for one of its actions.