core: Add English translations for copy/paste context menu

Author: kjarosh

core/assets/texts/en-US/context_menu.ftl

@@ -5,3 +5,8 @@ context-menu-back = Back
 context-menu-quality-low = Quality: Low
 context-menu-quality-medium = Quality: Medium
 context-menu-quality-high = Quality: High
+context-menu-cut = Cut
+context-menu-copy = Copy
+context-menu-paste = Paste
+context-menu-delete = Delete
+context-menu-select-all = Select All