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.h

@@ -52,14 +52,14 @@ class ResourcePage : public QWidget, public BasePage {
 
     [[nodiscard]] bool setCurrentPack(ModPlatform::IndexedPack);
     [[nodiscard]] auto getCurrentPack() const -> ModPlatform::IndexedPack;
-
     [[nodiscard]] auto getDialog() const -> const ResourceDownloadDialog* { return m_parent_dialog; }
-
     [[nodiscard]] auto getModel() const -> ResourceModel* { return m_model; }
 
    protected:
     ResourcePage(ResourceDownloadDialog* parent, BaseInstance&);
 
+    void addSortings();
+
    public slots:
     virtual void updateUi();
     virtual void updateSelectionButton();