core: Add padding members to class structures

This gives us flexibility for soft ABI breaks in future if needed, after
we lock down the ABI entirely.
core: Mark all API as stable and impose API and ABI stability guarantees

It’s time for libgdata to grow up and start guaranteeing some stability
for its users. In this vein, it’s already been API stable for around 3
years (since 0.9.1) — it’s about time that was made explicit.

http://upstream-tracker.org/versions/libgdata.html is quite interesting.