feat: add mod index updating to ModDownloadTask

This makes ModDownloadTask into a SequentialTask with 2 subtasks:
Downloading the mod files and updating the index with the new
information.

The index updating is done first so that, in the future, we
can prompt the user before download if, for instance, we discover
there's another version already installed.

launcher/ui/pages/modplatform/ModPage.cpp

@@ -150,7 +150,7 @@ void ModPage::onModSelected()
     if (dialog->isModSelected(current.name, version.fileName)) {
         dialog->removeSelectedMod(current.name);
     } else {
-        dialog->addSelectedMod(current.name, new ModDownloadTask(version.downloadUrl, version.fileName, dialog->mods));
+        dialog->addSelectedMod(current.name, new ModDownloadTask(current, version, dialog->mods));
     }
 
     updateSelectionButton();