Implement a new spelltype for abilities that channel

[abosma]

Currently implementing a channeled spell/buff requires creating all the event handlers yourself per spell/buff. Creating a channel spelltype that handles cancelling on damage/move/stun would help implementing these types of abilities a lot.

[abosma]

The Channel Spelltype would probably implement most of the things that Spell/Buff has, but it adds a few things like: InterruptableByDamage, InterruptableByMovement, InterruptableByStun.

Together with that it could contain a OnAbilityInterrupted in cases like Darius' Noxian Guillotine or Gragas' Drunken Rage.