Several improvements for DebuggerFrontEnd

- turned `FlxG.debug.show()`, `FlxG.debug.hide()` and `FlxG.debug.toggle()` into a single property named `FlxG.debug.visible` which is a lot cleaner and makes more sense
- improved docs of `DebuggerFrontEnd` and `FlxDebugger`
- removed `DEBUGGER` prefix from debugger layouts as they are now located in `FlxDebugger` where this is obvious