Symplify freeing memory by better exploiting glib functions

Mostly g_clear_object and g_clear_pointer. To find the occurrences I did "git grep -C1 -n ' = NULL'" and iterated over all the results that looked suspicious.

This is just a big cleanup, but my intention was to have zero functional changes. I carefully went through the diff, but this is a bit of monkey work, so I might have slipped some change.

I have not looked into backend code because it is a bit too much, and some of the formats might be worth some discussion.