Merge branch '1.8/melee_proc_refactor' into 'master'

Refactor melee hit proc

1. add two callbacks: callbackOnMeleeProject used for effects similar to melee-project and should be triggered before weapon's on-kill effects; callbackOnMeleeHitProcs used by the victim to trigger on-hit effects
2. move many melee procs in attackTargetHitProcs into related talents/effects by callback and thus greatly simplify this function
3. remove the obselete attributes "damage_backfire","movespeed_on_hit"
4. remove several unique talent attributes "shattering_impact", "knockback_on_hit", "onslaught"
5. slightly reorders the proc order, which should not make much difference though.

See merge request !789