Make sure a channel -> selection -> channel roundtrip never does any
gamma conversion.

In gimp_channel_duplicate(), make sure a created channel has the
right format, and the right data. Fixes selection -> channel.

When switching off quick mask, call gimp_item_to_selection() instead
if gimp_selection_load(), the latter was implementing a shortcut which
is now wrong.

Remove gimp_selection_load() which is now unused.

Unrelated: also remove gimp_selection_save(), it was an obvious
3-liner used only twice.