[FIX] point_of_sale: expose ClientListScreenWidget

Allow easy modification of ClientListScreenWidget by other modules. If
you get creative you can actually get to it without this, but it's not
clean and needlessly complicated.

opw-669686

addons/point_of_sale/static/src/js/screens.js

@@ -1961,6 +1961,7 @@ return {
     NumpadWidget: NumpadWidget,
     ProductScreenWidget: ProductScreenWidget,
     ProductListWidget: ProductListWidget,
+    ClientListScreenWidget: ClientListScreenWidget,
 };
 
 });