Gui: clean-up Selection API

Replace the int of the SubType of SelectionChanges with an enum class.
The meaning of it is nowhere documented and some magic numbers like 0,1,2 are used in several places in the code.

src/Gui/TaskElementColors.cpp

@@ -487,7 +487,8 @@ void ElementColors::on_elementList_itemEntered(QListWidgetItem *item) {
     }
     Selection().setPreselect(d->editDoc.c_str(),
             d->editObj.c_str(), (d->editSub+name).c_str(),0,0,0,
-            d->ui->onTop->isChecked()?2:1);
+            d->ui->onTop->isChecked() ? Gui::SelectionChanges::MsgSource::TreeView
+                                      : Gui::SelectionChanges::MsgSource::Internal);
 }
 
 void ElementColors::on_elementList_itemSelectionChanged() {