Fix loading extension path from project config (#2956)

Extension path was getting saved under the "extensionsPath" key
but getting read from the "extensionsFolder" key.

Read the value back from the written extensionsPath key.

(cherry picked from commit 647d240)

src/tiled/project.cpp

@@ -116,7 +116,7 @@ bool Project::load(const QString &fileName)
 
     const QJsonObject project = document.object();
 
-    mExtensionsPath = absolute(dir, project.value(QLatin1String("extensionsFolder")).toString(QLatin1String("extensions")));
+    mExtensionsPath = absolute(dir, project.value(QLatin1String("extensionsPath")).toString(QLatin1String("extensions")));
     mObjectTypesFile = absolute(dir, project.value(QLatin1String("objectTypesFile")).toString());
     mAutomappingRulesFile = absolute(dir, project.value(QLatin1String("automappingRulesFile")).toString());