Add bindings for gst_buffer_foreach_meta()

Needs two variants: one for mutable buffers and one for immutable ones. The callback should probably look like

FnMut(&BufferRef, &MetaRef) -> glib::Continue;
FnMut(&mut BufferRef, &mut MetaRef) -> (bool, glib::Continue);

or similar. The bool would cause the meta to be removed. Alternatively it could be some enum that handles the 4 different cases (continue/stop, remove/keep).