[IMP] point_of_sale: initial commit for the fullscreen mode.

[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.