docs: Add missing '\since' attributes to types and functions that have been added in versions later than 0.4.0

This adds \since tags to all types and functions that were missing them, to correct the gir where it is currently being patched downstream for the Rust bindings.

Unfortunately I don't think the doxygen-to-gtkdoc converter can currently handle properties and signals, so those are still missing the correct version in the generated .gir file.