refactor: extract common code in mod pages and model

This creates a hierarchy in which ModPage and ModModel are the parents
of every mod provider, providing the basic functionality common to all
of them.

It also imposes a unique .ui file (they were already equal before, just
duplicated basically) on all mod providers.

launcher/ui/pages/modplatform/ModPage.h

@@ -0,0 +1,57 @@
+#pragma once
+
+#include <Application.h>
+#include <QWidget>
+
+#include "modplatform/ModIndex.h"
+#include "tasks/Task.h"
+#include "ui/pages/BasePage.h"
+#include "ui/pages/modplatform/ModModel.h"
+
+class ModDownloadDialog;
+
+namespace Ui {
+class ModPage;
+}
+
+class ModPage : public QWidget, public BasePage {
+    Q_OBJECT
+
+   public:
+    explicit ModPage(ModDownloadDialog* dialog, BaseInstance* instance);
+    virtual ~ModPage();
+
+    inline virtual QString displayName() const override = 0;
+    inline virtual QIcon icon() const override = 0;
+    inline virtual QString id() const override = 0;
+    inline virtual QString helpPage() const override = 0;
+
+    inline virtual QString debugName() const = 0;
+    inline virtual QString metaEntryBase() const = 0;
+
+    virtual bool shouldDisplay() const override = 0;
+
+    void openedImpl() override;
+    bool eventFilter(QObject* watched, QEvent* event) override;
+
+    BaseInstance* m_instance;
+
+   protected:
+    virtual void onModVersionSucceed(ModPage*, QByteArray*, QString) = 0;
+
+    void updateSelectionButton();
+
+   protected slots:
+    void triggerSearch();
+    void onSelectionChanged(QModelIndex first, QModelIndex second);
+    void onVersionSelectionChanged(QString data);
+    void onModSelected();
+
+   protected:
+    Ui::ModPage* ui = nullptr;
+    ModDownloadDialog* dialog = nullptr;
+    ModPlatform::ListModel* listModel = nullptr;
+    ModPlatform::IndexedPack current;
+
+    int selectedVersion = -1;
+};