v0.6 Steam leaderboards + new features for custom levels

This update adds Steam leaderboards for official and custom levels. You can now compare your scores with your Steam friends, and see your global rank for any level. Leaderboards can be accessed from the level selection menu.

You get one entry on any leaderboard per Steam account, meaning that for a given level, your leaderboard hi-score will always be the highest value found in your all your Nebula Flow profiles.
Any previous hi-score you did will be uploaded to the relevant leaderboard when first opening it. Any new hi-score will be uploaded immediately when reaching the end of a level.

Following these new features, your custom level scores are now saved into your Nebula Flow profile. You can now set a difficulty value (from 1 to 10) for your custom levels in the level editor. This value will be displayed as a number of stars (from half a star up to five stars) when you select a custom level. For leaderboard consistency, custom levels can no longer be modified aside from their metadata (title, artist, map author, difficulty, and source) once they have been uploaded to the Workshop.

Changelog:

• Added global/friends Steam leaderboards for official and Workshop levels.
• Difficulty value (stars) can now be set for custom levels in level editor.
• Custom levels now display best score, difficulty value and more.
• Some changes in level selection menus in accordance with current and upcoming features.
• Fixed combo counter sometimes being incremented in the wrong order by flying numbers.
• Fixed potential crash in level editor.