alias GenericItem#category to GenericItem#icon

that's what it's called in the sitemap builder, and what it's effectively
for now

Signed-off-by: Cody Cutrer <cody@cutrer.us>

lib/openhab/core/items/generic_item.rb

@@ -308,7 +308,7 @@ def label=(value)
         end
 
         # @!attribute [rw] category
-        # The item's category.
+        # The item's category (icon).
         # @return [String]
         def category=(value)
           modify do
@@ -319,6 +319,8 @@ def category=(value)
             set_category(value)
           end
         end
+        alias_method :icon, :category
+        alias_method :icon=, :category=
 
         # @!attribute [rw] tags
         #   The item's tags