Initial commit

Half-baked state. Most of the UI is mostly done, but the integration
with Grist (or any other data provider) is still missing.

data_resources/cart_entry.gd

@@ -0,0 +1,10 @@
+extends Resource
+class_name CartEntry
+
+var cart_index: int = -1
+var article_info: ArticleInfo
+var weight_grams: int
+var price_cents: int
+
+func _init(p_article_info: ArticleInfo) -> void:
+	article_info = p_article_info