Mocap 1.4-alpha-3

[1.4-alpha-2 -> 1.4-alpha-3]
-It's now possible to set playback modifiers for "/mocap playback start" command.
-Added command to set playback modifiers - "/mocap playback modifiers".
-Added support for custom NBT data with "player_as_entity" modifier.
-Removed position offset arguments from "/mocap scenes add_to".
-Updated scene format - it's now JSON-based.
-Added setting "pretty_scene_files" which makes scene JSON files look pretty (true by default).
-Added "minecarts" entity filter group.
-Added error message when trying to start empty root scene.
-Renamed entity filter group "empty" to "none".
-Renamed "/mocap scenes info/list" scene arguments from "name" to "scene_name".
-Split "player_info" modifier into "player_name" and "player_skin".
-Replaced "recording_sync" setting with "/mocap misc sync" command.
-Added "for_all_players" argument to "/mocap playback stop_all".
-Renamed "from_..." to "skin_from_..." simple modifiers in "playback start" and "scene add_to" commands.
-It's now possible to add player_as_entity modifier using simple modifiers (thanks to Narfu for suggestion).
-Fixed error when saving "change item" action for non-player entity.
-Fixed issue with single entity filter instances not working.
-Fixed directories with a proper name showing up on a list of recordings or scenes.
-Fixed server crashing when trying to start playback with player name longer than 16 characters (thanks to SPY for reporting).
-Fixed fake players with empty name having tiny gray bar above their head.