BitmapLog window: add cache viewer feature, more improvements

- added FlxG.bitmap.viewCache() (or in the console: "viewCache" / "vc")
- graphics now have a bounding box
- added the ability to name graphics
- now showing the dimensions and file size of bitmaps
- added FlxStringUtil.formatBytes()
- better zooming
- Debugger windows are now put on top while being resized
- use signals for state switch listeners in Tracker.hx / Watch.hx