Layers can be negative on all targets (getting rid of HXP.BASELAYER)

Changed _layerList to be an `IntMap` so now we can have negative layers on neko too. Should also be slightly more performant since Arrays are dense, meaning if you had used layers 10 and 90 you would have had an Array of (90-10) elements with nulls in between.

Tested this with https://github.com/azrafe7/HaxePunk-minitests but if you can try it further would be nice.

Layer value is not showing in the WATCH_LIST, probably this should only be getProperty for all targets here (https://github.com/HaxePunk/HaxePunk/blob/master/com/haxepunk/debug/Console.hx#L961-L965)