Level save improvements

[aleksigron]

• Levels are part of asset library and are identified with UIDs
• Levels are opened from the asset browser view
• FilePickerDialog has option for using asset directory relative paths
• File handling code is removed from LevelSerializer, it now only works to serialize from and to string types.
• Level loading and saving code is removed from World
• Last opened level is opened on editor boot
• Some improvements to Array and StringRef types
• Other minor refactoring and renames