Add button IDs for Dualshock 4
Merge pull request #989 from massivedanger/ps4-controller-support

Add button IDs for Dualshock 4
Remove unnecessary @default from docs
FlxSprite / FlxTilemap: fix the order of drawDebug(), fixes #988

needs to happen *after* the actual draw() logic
FlxSignalTest: add null tests back
FlxSignal code style
Fix tests

no return values for add() (yet)
Make it easier to play FlxSplash in debug mode
FlxGame: take state visibility into account before drawing or updating anything
FlxSound: survive -> persist

Makes it consistent with CachedGraphics.persist
Got rid of single var tweens, MultiVarTween -> VarTween

also
  - added some safety checks to VarTween.tween()
  - removed an unnecessary null check from the update() loop
FlxText: fix bug where 0-length FlxText's show improper backgrounds in FlxUIInputText extension
Merge branch 'dev' of http://www.github.com/HaxeFlixel/flixel into dev
Fix some issues with 64c7a418b8f35f8badca143b42d407b8c38c0c31

thanks @gamedevsam
FlxText: Added more elaborate comment for bug fix
Fixed capitalization in comment
Merge branch 'bugfix_flxtext' of https://github.com/larsiusprime/HaxeFlixel into dev
FlxText: nasty workaround for initialization with empty strings on flash
Merge branch 'dev' of https://github.com/HaxeFlixel/flixel into dev