Implement legacy forge button!

Many refactors of the task system.
Progress dialog now accepts generic ProgressProvider objects

MultiMC.cpp

@@ -282,6 +282,7 @@ void MultiMC::initHttpMetaCache()
 	m_metacache->addBase("assets", QDir("assets").absolutePath());
 	m_metacache->addBase("versions", QDir("versions").absolutePath());
 	m_metacache->addBase("libraries", QDir("libraries").absolutePath());
+	m_metacache->addBase("minecraftforge", QDir("mods/minecraftforge").absolutePath());
 	m_metacache->Load();
 }