Taskbar: Sort parent categories before their children

The category sort treated a longer path as ordering ahead of its prefix,
so a nested category like "Games/Puzzles" was sorted before its parent
"Games". create_category_menu() then looked up the not-yet-created
parent menu with .value() on an empty optional, crashing the Taskbar on
startup.

Make less_than_ignore_hotkey() sort the shorter string first when one is
a prefix of the other, so a parent category always precedes its
children. This also simplifies create_category_menu() since the parent
is guaranteed to already exist.

Userland/Services/Taskbar/main.cpp

@@ -103,7 +103,9 @@ static bool less_than_ignore_hotkey(ByteString const& a, ByteString const& b)
         ++a_it;
         ++b_it;
     }
-    return a_it != a.end();
+    // When one string is a prefix of the other, sort the shorter one first so
+    // that a parent category ("Games") precedes its children ("Games/Puzzles").
+    return a_it == a.end() && b_it != b.end();
 }
 
 ErrorOr<Vector<ByteString>> discover_apps_and_categories()
@@ -159,11 +161,6 @@ ErrorOr<NonnullRefPtr<GUI::Menu>> build_system_menu(GUI::Window& window)
             parent_menu = system_menu;
         } else {
             parent_menu = app_category_menus.get(parent_category).value();
-            if (!parent_menu) {
-                create_category_menu(parent_category);
-                parent_menu = app_category_menus.get(parent_category).value();
-                VERIFY(parent_menu);
-            }
         }
         auto category_menu = parent_menu->add_submenu(String::from_byte_string(child_category).release_value_but_fixme_should_propagate_errors());
         auto category_icon_path = category_icons->read_entry("16x16", category);