add set_metadata, get_metadata script functions

doc/function/set_metadata.txt

@@ -0,0 +1,16 @@
+Function: set_metadata
+
+--Usage--
+> set_metadata(input: image, metadata: string)
+
+Returns a copy of the input image with the metadata set to the given string.
+
+Metadata is only stored when an image is saved as a PNG.
+It is saved in a uncompressed iTXt chunk with the key "Description".
+If the image is saved as a JPG, the metadata is lost.
+
+--Parameters--
+! Parameter	Type				Description
+| @input@	[[type:image]]		The image that will be copied.
+| @metadata@	[[type:string]]		The metadata that will be stored in the copy.
+| @set@		[[type:set]]		The active set. This can be omitted since 'set' is a predefined variable.