First fully functional POS.

For full background of this effort please see this discussion.

Important points to note:

1. Frappe Books, unlike ERPNext is focused on single user use case. As a result there is no user authentication layer or multi-user support.
2. Due to point 1, we don't have POS profile, POS opening, POS closing etc as in case of ERPNext.
3. The scope & objective of POS UI that am working on is very limited. It is to create a UI for the books users to use Frappe Book as POS. That is give him/her the ability to quickly create an invoice via a POS like UI and submit the invoice with payment entry in one go. (Payment in one go, is yet to be implemented)
4. Currently, though Frappe Books is good enough for a single owner boutique or retail shop, the lack of a POS like interface results in much friction and requires more clicks than needed.
5. Just having an alternative UI for invoices with payment automatically created as the invoice is submitted enables faster invoicing for some use-cases.
6. For a more elaborate use case, we will need to further extend Frappe Books, to have concepts like user, roles, auth etc. Which however may not be within the scope of Frappe Books itself.

See the POS in action below: