data binding for owned item properties

Additions:
- now also text, number, and range input fields of owned items can
  be bound to HTML (input) elements of the char sheet

Changes:
- moved prototype handle for checkboxes to own function
- replaced some static HTML elements in item list with input fields
- item-num-val HTML elements (e.g. item quantity) now have a max-width

src/scss/components/_items.scss

@@ -36,6 +36,7 @@
 
     .item-num-val {
         text-align: center;
+        max-width: 2.5em;
     }
 
     .item-description {