Some big work on the inventory system

turn the show into a call screen
replace all return actions with specific local actions (WIP, more changes to come)
make the global variables at least dynamic
make the mode parameter a parameter of the label (so, a dynamic variable too, as a result)
replace the bigger actions with local labels
tweak text entries, comparisons, iterations... details

game/scripts/interface/gifts.rpy

@@ -1,7 +1,5 @@
 label gift_menu:
-    $ inventory_mode = 1
-    $ gui.in_context("inventory_menu")
-    $ inventory_mode = 0
+    $ gui.in_context("inventory_menu", inventory_mode=1)
     return
 
 label give_gift(text, gift):