Add selection tracking to selector with selected and deselected methods

lib/level/selector.lua

@@ -1,5 +1,6 @@
 --- @class Selector
 --- @field id Id | nil
+--- @field lastId Id | nil
 --- @field locked boolean
 local selector = {}
 selector.__index = selector
@@ -15,6 +16,7 @@ function selector:select(characterId)
 end
 
 function selector:update(dt)
+    self.lastId = self.id
     if self.locked or not Tree.controls:isJustPressed("select") then return end
 
     local mousePosition = Tree.level.camera:toWorldPosition(Vec3 { love.mouse.getX(), love.mouse.getY() }):floor()
@@ -53,6 +55,16 @@ function selector:unlock()
     self.locked = false
 end
 
+--- If a character was selected during this tick, returns its Id
+function selector:selected()
+    if self.id and self.id ~= self.lastId then return self.id end
+end
+
+--- If a character was **de**selected during this tick, returns its Id
+function selector:deselected()
+    if not self.id and self.lastId then return self.lastId end
+end
+
 return {
     new = new
 }