refactor(RD): clear up sorting methods

This refactors the sorting methods to join every bit of it into a single
list, easing maintanance. It also removes the weird index contraint on
the list of methods by adding an index field to the DS that holds the
method.

Lastly, it puts the available methods on their respective API, so other
resources on the same API can re-use them later on.

Signed-off-by: flow <flowlnlnln@gmail.com>

launcher/ui/pages/modplatform/ResourcePage.cpp

@@ -103,6 +103,17 @@ void ResourcePage::setSearchTerm(QString term)
     m_ui->searchEdit->setText(term);
 }
 
+void ResourcePage::addSortings()
+{
+    Q_ASSERT(m_model);
+
+    auto sorts = m_model->getSortingMethods();
+    std::sort(sorts.begin(), sorts.end(), [](auto const& l, auto const& r) { return l.index < r.index; });
+
+    for (auto&& sorting : sorts)
+        m_ui->sortByBox->addItem(sorting.readable_name, QVariant(sorting.index));
+}
+
 bool ResourcePage::setCurrentPack(ModPlatform::IndexedPack pack)
 {
     QVariant v;