Add keypad layout with 0 in the middle

[sergiuser1]

This is the standard layout for number keypads, here's a screenshot of how the Google Keyboard looks: And this is the new keyboard:

This PR adds that option, while keeping the default with the separator in the middle, and also migrating the config changes for existing users. The only thing missing is the new translations, for all other languages besides English.

[A-Sattari]

Having 0 in the middle for sure offers a better user experience. @sal0max is there a reason why this PR is not merged?
Also, I would recommend removing the arithmetic operators too. They do not really server a big purpose in a currency convertor app. It just makes the view cluttered