clapper: Allow peeking in Vala

Vala does better job at handling objects without increased reference than
interpreted languages, so its safe to expose list "peek" functions to it.

src/lib/clapper/metadata/Clapper-0.0.metadata

@@ -1,6 +1,6 @@
 // Skipped by GI, but Vala can handle it fine
-//init_get_option_group skip=false
 *_FORMAT skip=false
+*.peek_* skip=false
 
 // Init func compatibility
 init.argv unowned