[core] Consolidate file source keys in the generic FileSource header

Thus we do not need to include specific headers to use `{set|get}Property`
API with key constants.

render-test/file_source.cpp

@@ -20,7 +20,7 @@ ProxyFileSource::ProxyFileSource(std::shared_ptr<FileSource> defaultResourceLoad
     assert(defaultResourceLoader);
     if (offline) {
         auto dbfs = FileSourceManager::get()->getFileSource(FileSourceType::Database, options);
-        dbfs->setProperty("read-only-mode", true);
+        dbfs->setProperty(READ_ONLY_MODE_KEY, true);
     }
 }