[ADD] 'pos_product_template'

Propose a module to select variant of template.

Main features:
- products variant are now hidden from the POS (only one template is displayed);
- it is still possible to scan variant;
- an extra screen is displayed to select variant when a template is selected;

see https://www.youtube.com/watch?v=sCZr39NxSlU&feature=youtu.be

Review and test are welcome.
same here: this.template_name seems unsued