Add ModpackUpdatePromptDisabled setting

When creating an instance of a modpack that's already associated with
another instance, the user gets asked if they want to update that
instance instead. This commit introduces a setting to suppress the
prompt and directly create the new instance.

Signed-off-by: Kationor <n96211028@gmail.com>

launcher/Application.cpp

@@ -662,6 +662,7 @@ Application::Application(int& argc, char** argv) : QApplication(argc, argv)
         // Minecraft mods
         m_settings->registerSetting("ModMetadataDisabled", false);
         m_settings->registerSetting("ModDependenciesDisabled", false);
+        m_settings->registerSetting("ModpackUpdatePromptDisabled", false);
 
         // Minecraft offline player name
         m_settings->registerSetting("LastOfflinePlayerName", "");