static keys - items in inventory will save their assigned key

precursor to cleaning up/modularising inventory display, instead of needing to iterate through every item held to find unique copies, we can just check if the button pressed corresponds to any entity's Key {} index

src/components.rs

@@ -468,9 +468,20 @@ pub struct IdentifiedItem {
     pub name: String,
 }
 
+#[derive(Component, Debug, Serialize, Deserialize, Clone)]
+pub struct Stackable {}
+
 #[derive(Component, Debug, Serialize, Deserialize, Clone)]
 pub struct EquipmentChanged {}
 
+#[derive(Component, Debug, Serialize, Deserialize, Clone)]
+pub struct WantsToRemoveKey {}
+
+#[derive(Component, Debug, Serialize, Deserialize, Clone)]
+pub struct Key {
+    pub idx: usize,
+}
+
 #[derive(PartialEq, Copy, Clone, Serialize, Deserialize)]
 pub enum BurdenLevel {
     Burdened,
@@ -654,6 +665,9 @@ pub struct InBackpack {
     pub owner: Entity,
 }
 
+#[derive(Component, Serialize, Deserialize, Clone)]
+pub struct WantsToAssignKey {}
+
 #[derive(Component, Debug, ConvertSaveload)]
 pub struct WantsToPickupItem {
     pub collected_by: Entity,