Add achievement item UI and logic for displaying achievements

ui/achievements/achievement_item.gd

@@ -0,0 +1,28 @@
+extends Control
+
+@onready var icons = {
+    "1c": $Row/Icon1c,
+    "1g": $Row/Icon1g,
+    "3c": $Row/Icon3c,
+    "3g": $Row/Icon3g,
+    "6c": $Row/Icon6c,
+    "6g": $Row/Icon6g,
+    "10c": $Row/Icon10c,
+    "10g": $Row/Icon10g,
+}
+
+@onready var event_label: RichTextLabel = $Row/Label
+
+## Shows this achievement event with the matching points/state icon.
+## event_type is "c" for completed or "g" for revealed but not completed.
+func open(level: int, event_type: String, event_text: String) -> void:
+    var icon_key := str(level) + event_type.to_lower()
+    if not icons.has(icon_key):
+        push_warning("Unknown achievement icon: %s" % icon_key)
+        return
+
+    for key: String in icons:
+        icons[key].visible = key == icon_key
+
+    event_label.text = event_text
+    visible = true