[IMP] point_of_sale: add an error sound to the error popups
[IMP] point_of_sale: initial commit for the fullscreen mode.
The fullscreen option will detect if the pos is displayed in fullscreen
and if not will tell the user how to set it up for fullscreen, or propose
to go fullscreen with the HTML5 Built-in API.

The HTML5 API requires a click from the user to allow fullscreen, so the
popup will pop everytime unless the user has setup kiosk-mode or native
application emulation which makes the app start in real fullscreen.
[IMP] point_of_sale: a little explaination of how the barcode pattern system works
[IMP] point_of_sale: improved rendering speed of the partner list
[FIX] point_of_sale: order invoicing was broken
[FIX] point_of_sale: it was impossible to delete a pos session
[FIX] point_of_sale: it was impossible to delete a pos session
Merge branch 'master' into master-pos-small-imps-of-july-fva
[IMP] point_of_sale: show traceback popup when there is a server error when posting orders (which should NEVER happen !)
[IMP] point_of_sale: add debug popup that displays unsent order data, for when localStorage is hard to inspect
[IMP] point_of_sale: add a debug option to delete all unsent orders
[IMP] point_of_sale: add a basic UI unit test based on the new 'Tour' module
[IMP] point_of_sale: forgot changes in this file in the previous commit that introduced unit tests
[IMP] point_of_sale: tests work better when they are loaded and run by the unit test system!
Merge branch 'master' into master-pos-small-imps-of-july-fva
[IMP] point_of_sale: made the server data loading more modular for easier development of point_of_sale extensions
[IMP] point_of_sale: report errors when loading the point of sale with a traceback and a nice popup
[IMP] pos_discount: a new pos extension that allows you to quickly give a global discount on the current order
Merge branch 'master' into master-pos-small-imps-of-july-fva
[IMP] point_of_sale: allow an order to remember what screen it was on, instead
of switching back to default screen.

This is used by the soon to be merged restaurant addon to create new orders
and display them at a particular screen.  However, this behaviour is more
confusing in the other cases, so I'll think of a better way to solve this.