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/MinecraftInstance.h

@@ -69,6 +69,8 @@ public:
 
 	virtual QString getNativePath() const = 0;
 
+	virtual QString getLocalLibraryPath() const = 0;
+
 	virtual QStringList processMinecraftArgs(AuthSessionPtr account) const = 0;
 
 	virtual JavaVersion getJavaVersion() const;