[MERGE] point_of_sale: hardware proxy reliability improvements

addons/hw_escpos/controllers/main.py

addons/hw_escpos/escpos/constants.py

addons/hw_escpos/escpos/escpos.py

addons/hw_proxy/controllers/main.py

addons/hw_scanner/controllers/main.py

addons/point_of_sale/static/src/css/pos.css

addons/point_of_sale/static/src/js/devices.js

addons/point_of_sale/static/src/js/models.js

addons/point_of_sale/static/src/js/widgets.js

addons/point_of_sale/static/src/xml/pos.xml