NOISSUE add instance-local library storage

Any libraries stored in $instanceroot/libraries/ will override
the libraries from MultiMC's global folders, as long as they are marked 'local'
in the json patch.

api/logic/minecraft/onesix/OneSixInstance.h

@@ -104,6 +104,8 @@ public:
 	QStringList getNativeJars() const override;
 	QString getNativePath() const override;
 
+	QString getLocalLibraryPath() const override;
+
 	QStringList processMinecraftArgs(AuthSessionPtr account) const override;
 
 protected: