Removed redundant code

Added private helpers that hold the code shared between add- and append- functions in order to remove redundancy.

addBy-/appendBy- -Names, -StringIndices, -Indices, and -Prefix were edited.
Cleanup
Uncuddled braces

and moved private helpers to the bottom of the class
Merge pull request #959 from MSGhero/dev

Removed redundant code in FlxAnimationController