[IMP] account, point_of_sale: properly handle cashbox

When you use a cash journal in the point of sale, you can enable the
'Cash Control' option on your pos config to be able to count the money
in your cashbox before and after selling goods in the POS.

To better manage this behavior, we've made some improvements to handle
the starting and ending of a session.

First, instead of having just some cashbox lines set on the pos config
representing the default cash fund in the cashbox, we are using an
object of type cashbox as template. This allows to use the same default
config for multiple POS config without recreating it.

Then, we've added a currency on the cashbox, which is just computed
based on the currency referencing the cashbox.

When a POS session is open, the content of the cashbox is the same as
the last closed session, to represent how a real cashbox works.

We also always have the possibility to set the default cashbox of the
POS config at the begining of a session.

TASK-ID: 1934784