[Companion] Qualify all translation strings (#5557)

* [Companion] Qualify all remaining QObject translations with proper class/object names; Centralize some common strings into constants.h.

* [Companion] Replace all instances of some common strings with macros from constants.h.

* [Companion] Fix translation warnings from lupdate.

companion/src/modelslist.cpp

@@ -678,7 +678,7 @@ void TreeModel::refresh()
           if (!defaultCategoryItem) {
             defaultCategoryItem = rootItem->appendChild(0, -1);
             /*: Translators do NOT use accent for this, this is the default category name on Horus. */
-            defaultCategoryItem->setData(0, QObject::tr("Models"));
+            defaultCategoryItem->setData(0, tr("Models"));
           }
           categoryItem = defaultCategoryItem;
         }