Change USB device identifier

[robbi5]

This PR changes the USB device identifier to actually use leading zeros, if the usb vendor id / product id uses them - and prefer that over the device serial number by default.

It also includes some UX improvements by moving the device identifier textfield next to the choose device button (to make them seem more related), changing the compatibility mode switch to a checkbox and offering users to "pin" the device by click, to use the device serial number again.

[robbi5]

really preferring the device vendorId/productId over serial number happened in 97f5821 - this PR has to rebased if that got merged.

[raphaelm]

I like it, the "pin" is a nice tough although it's probably not intuitive for most users to understand why it is there… Still, for a power-user feature it's nice.

I'm mainly worried currently about the following: The motivation behind preferring product ID over serial number was that, if I have 3 cashdesks and 3 printers, I no longer need to care which printer I plug into which cashdesk …

… but have we tested that this is actually true? If I grant pretixPRINT permission for a USB device, does the Android system persist the permission based on the serial number or based on the product id? In other words, if I switch between printers of the same model, do we need to go through pretixPRINT again anyways to get the permission? Or would it just work?

[raphaelm]

… but have we tested that this is actually true? If I grant pretixPRINT permission for a USB device, does the Android system persist the permission based on the serial number or based on the product id? In other words, if I switch between printers of the same model, do we need to go through pretixPRINT again anyways to get the permission? Or would it just work?

I have tested it: It does not work. I have used two Epson TM-m30ii with the same product/vendor ID. After connecting one printer, I closed the app, switched to the other printer and tried to print a receipt, the printer was not found. Only after I re-selected it in pretixPRINT and confirmed the system-level USB permission again, it worked.

Does this PR even make sense then? :(

[raphaelm]

Okay, at least switching back works. So if I authorize all my printers on all my devices once, I can then switch, so there's a little upside to this PR. Still, the user experience is weird if the config shows the correct IDs and it says "printer not found". Should we try to detect this case and give a better error message?