Add iterator for groups (all, dead, alive, exists)
iteratorAlive: check if alive AND exists
Rename FlxGroupIterator to FlxTypedGroupIterator
Fix typo in comment `members` to `member`
Inlined `iterator()`
Change `iteratorAlive`, `iteratorDead` and `iteratorExists` to properties to get rid of `()`
Add `forEach`, `forEachAlive`, `forEachDead` and `forEachExists`
Delete old FlxGroupIterator.hx file. Missing in last commit
Fix bug: You couldn't iterate over an element that had called remove without slice
Replace `T` with `FlxBasic`
@crazysam suggestion
Fix parameter signature of forEachDead and forEachExists
Generics iterator
Custom filter for iterator
Inlined specific iterators (alive, dead, exists)
Add missing file
Removed useless iterators
Merge pull request #719 from Masadow/groupIterator

Add iterator for groups (all, dead, alive, exists), closes #704