Import const-correctness changes from preliminary 1.18 branches
Import const-correctness changes from gstreamer!730 (merged) and gst-plugins-base!1005 (merged)
Please hold off merging this until those are merged, at that point I really want to clean up history and put commit hashes in the import commits (especially since these are not based on a tag release).
Edited by Marijn Suijten