Add functions hero:is/set_invincible()

Created by: christopho

We need to be able to make the hero temporarily invincible, optionally with his sprites blinking for a few seconds, like after being hurt.

hero:is_invincible() hero:set_invincible([invincible, [delay]]) hero:is_blinking() hero:set_blinking([blink, [delay]])

Being invincible and having the sprites blinking are two independent properties because sometimes we need one property without the other. Example: an equipment item that makes the hero temporarily invicible.