[IMP]: point_of_sale UI: `NumberPopup` and `NumberBuffer` can pre-select input...

[IMP]: point_of_sale UI: `NumberPopup` and `NumberBuffer` can pre-select input which will change the value upon triggering a key

The user doesn't need to delete the current input before changing it. He can now write
hiw new value which will erase the current one.
The guest popup from the `pos_restaurant` module and the global discount popup from the
`pos_discount` module benefit from this and have their input pre-selected.

Before: when opening the guest or discount popup, the user always has to delete the curent
value before changing it. Let's say that the current value was `5`, the user needs to either
press the `Delete` or `Backspace` button/key before entering his new value.

After: the user doesn't need to press the `Delete` or `Backspace` button/key and can now directly
enter his new value

Task ID: 2419637