Refactoring and implementation of the LaunchProjectileEvent

[RedNesto]

SpongeAPI | SpongeCommon

This PR implement its SpongeAPI counterpart.

When is the event fired ?

Here are each cases I tested:

• A non-player entity shoots a projectile (skeletons shooting arrows, llamas spitting, ghasts shooting fireballs, snowmans shooting snowballs, blaze shooting small fireballs)
• A player shoots arrows (with a bow heh)
• A player throws projectiles (snowballs, eggs, potions - splash and lingering -, enderpearls, eyes of ender, using fishing rod, experience bottle)
• A dispenser dispenses projectiles (arrows, snowballs, eggs, potions - splash and lingering -, experience bottle)
• A plugin shoots a projectile
• A player uses a firework.
• A dispenser dispenses a firework.
• A Wither attacks.
• The dragon shoots a fireball

Tell me if you see a case of projectile I missed.

And there is the content of a test plugin

Note: It seems that there are two projectile-related EventContextKeys, PROJECTILE_SOURCE and THROWER, from what I saw, only one would be necessary, I may address this issue later if it is considered as one.
Fixed in 5d0184d

[RedNesto]

~wip

[gabizou]

This will need to be re-done and re-target api-8. Bleeding is no longer a valid branch.