GH-1314 add UI for custom minecraft jar addition

Also changes the text of the jar mod addition button.
It should be clearer what it does and hopefully will not confuse
as many people.

application/MultiMC.cpp

@@ -501,9 +501,6 @@ MultiMC::MultiMC(int &argc, char **argv) : QApplication(argc, argv)
 
 		m_settings->registerSetting("UpdateDialogGeometry", "");
 
-		// Jar mod nag dialog in version page
-		m_settings->registerSetting("JarModNagSeen", false);
-
 		// paste.ee API key
 		m_settings->registerSetting("PasteEEAPIKey", "multimc");