Ability to change a resource UID from API

* Works for text, binary and imported resources
* Allows better clean up of duplicate files.

TODO (future PRs):

* Use this API for assigning new UIDs to copied files.
* Use this API for UID conflict on FS scanning (if more than one file has the same UID, the newer one(s) should get assigned a different UID).

core/io/resource_importer.h

@@ -32,6 +32,7 @@
 #define RESOURCE_IMPORTER_H
 
 #include "core/io/resource_loader.h"
+#include "core/io/resource_saver.h"
 
 class ResourceImporter;
 
@@ -149,4 +150,11 @@ public:
 
 VARIANT_ENUM_CAST(ResourceImporter::ImportOrder);
 
+class ResourceFormatImporterSaver : public ResourceFormatSaver {
+	GDCLASS(ResourceFormatImporterSaver, ResourceFormatSaver)
+
+public:
+	virtual Error set_uid(const String &p_path, ResourceUID::ID p_uid) override;
+};
+
 #endif // RESOURCE_IMPORTER_H