libseed-list handling gpointer args/returns

[Jonatan Liljedahl <lijon kymatica com>]

many functions in GLib take gpointer as args or return values, for 
example GLib.async_queue_push(queue, data).

but this doesn't work currently, it gives errors like "ArgumentError 
Unable to make argument 2 for function: async_queue_push."

There is a hack in seed_gi_make_argument() that allows strings to be 
passed, but this doesn't seem to work either (async_queue_pop() don't 
seem to return a pushed string, but undefined)

it should be possible and perhaps even simple to just wrap seed 
objects/values in these cases.

what function does the gvalue-to-seed conversion of the return value?

/Jonatan