Merge pull request 'Add the office background.' (#25) from feature/task22_programming_office into master

Reviewed-on: #25

inc/system/system.day.lua

@@ -0,0 +1,11 @@
+local _day_increase_handlers = {}
+
+function Day.increase()
+    Context.day_count = Context.day_count + 1
+    for _, handler in ipairs(_day_increase_handlers) do
+        handler()
+    end
+end
+function Day.register_handler(handler)
+    table.insert(_day_increase_handlers, handler)
+end