Fireballs and witherskulls can use the motion instead of the direction tag

This is probably the reason for:

• MC-4465

This bug is probably the reason for these three bugs:

• Fireballs can "jump" on slimeblocks (see attached movie)

• Fireballs "loose" their direction after reloading the world

• Summoned fireballs never fly like punched ones

Normally fireballs use the direction tag to determine where to fly. This has two effects:

• They get never slower

• They aren't affected by gravity

I found out, that even if you summon a fireball with direction:[0.0,0.0,0.0] and punch it, the fireball constantly sets its direction tag as its Motion tag. It appears also that while a fireball is flying its direction tag increases! Maybe this should get fixed too, because always calculating the motion based on the direction which increases the whole time and also setting the direction as motion seems pretty unefficient...

Anyways, appearently when you reload a world fireballs slow down very quick and then stay at the same position, same with summoned fireballs with a custom direction tag. And also when a fireball which has an increasing direction (so it was punched) hit a pushed slimeblock, but not when a fireball with a summoned custom direction hits a slimeblock!