gtkicontheme: More explicitly note ownership transfers of duplicated string
gtkicontheme: More explicitly note ownership transfers of duplicated string

I was hunting a memory leak and couldn't find it; at least I'm
pretty sure all of these are OK.  But document things better
for the future.

Also use g_hash_table_replace in one more case for consistency.

https://bugzilla.gnome.org/show_bug.cgi?id=649457

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