Proposition : model number handling (#4308)

* Try something for discussions

* Better alternative

* Typo

* Improve

* Change delimiter

* Move delimiter to space

* * Refactoring of CategorizedStorageFormat::load() and CategorizedStorageFormat::save() to support unused model slots
* Fix all unsafe usages of QString::toStdString().c_str()

* Fixed radio data import [Horus] (broken in 0f90ff0b65)

(cherry picked from commit cc12601a01)

* Added Q_OBJECT to TreeModel and some cosmetics

(cherry picked from commit f40e64c2aa)

* Cleanup

companion/src/mdichild.cpp

@@ -422,7 +422,7 @@ void MdiChild::modelAdd()
   model.category = categoryIndex;
   model.used = true;
   sprintf(model.filename, "model%lu.bin", (long unsigned)radioData.models.size()+1);
-  strcpy(model.name, tr("New model").toStdString().c_str());
+  strcpy(model.name, qPrintable(tr("New model")));
   radioData.models.push_back(model);
   radioData.setCurrentModel(radioData.models.size() - 1);
   setModified();