Five icons in the Layer dockable were being replaced by GTK defaults at
runtime. A "gimp-" prefix was added so that GIMP's version would always
be used. A few dialogues were fixed to use constants rather than
hardcoding the filename, to make it easier to update the icon in the
future.
Nobody has them anymore, and they are deprecated in GTK+ 3.x. This
also fixes all conflicting mnemonics except those I missed, but we can
fix them now.