ThickClient: avoid null values returned from iters in models (bgo#683359)

Some time ago a workaround was committed to avoid banshee crashes
when the values of model iterators were null [1]. The root of the
issue was that there was a small time-window in between the calls
to InsertNode() and SetValue() in which the iter could be queried
and its value would not be found. The way to fix this is to provide
the values in the same call that inserts the node, via the
InsertWithValues() method from the API.

[1] https://git.gnome.org/browse/banshee/commit/?id=d835de9ebc75bf937aa66d45909cb5eb119249e0

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>