[16.0][MIG] migrate pos_tare to 16.0

[robinkeunen]

This is a first pass at the migration from version 12 through 14 (#1206 ).

I'm having trouble testing the barcode behavior, how do you find out the check digit for an encoded tare barcode ?

Refactoring suggestion

The parameter iface_gross_weight_method lets you choose wether to input gross weight manually (manual) or through the scale (scale).

1. if manual is selected and iface_electronic_scale is false : you can enter the weight and tare through the numpad widget
2. if manual is selected and iface_electronic_scale is true : scale screen opens and you can input weight manually in the screen. The scale will never be read. => why open the scale screen if the scale is not read and everything can be done through the product screen
3. if scale is selected and iface_electronic_scale is false : the scale screen does not open and you can input the weight and tare through the widget => this seems inconsistent to me
4. if scale is selected and iface_electronic_scale is true : the scale screen opens, the scale is read.

I want to remove the iface_gross_weight_method altogether to simplify the configuration and the code. @legalsylvain what do you think about it ?