Big BIG refactor to make inventory use a model that's placed into the world, following a more sane MVC style.

meson.build

@@ -106,7 +106,7 @@ sources = [
   'gui/overlay_ui.cpp',
   'gui/ritual_ui.cpp',
   'gui/status_ui.cpp',
-  'gui/uisystems.cpp',
+  'inventory.cpp',
   'levelmanager.cpp',
   'lights.cpp',
   'map.cpp',
@@ -139,6 +139,7 @@ executable('runtests', sources + [
   'tests/easings.cpp',
   'tests/event_router.cpp',
   'tests/fsm.cpp',
+  'tests/inventory.cpp',
   'tests/levelmanager.cpp',
   'tests/lighting.cpp',
   'tests/loot.cpp',