Add achievements to help users discover features

[RodZill4]

No description provided.

[RodZill4]

Here's how to help with this PR:

• Achievements are defined in the material_maker/tools/achievements/achievements.gd file
• Each achievement is unlocked using the following code:

var achievements = get_node("/root/MainWindow/Achievements")
if achievements != null:
    achievements.unlock("achievement_id")