Keyboard mapping under Linux

According to the description on a well-known Chinese webshop, the programmable 4x5 mini-keyboard should work on Linux via USB.  The "plug and play without driver" indeed worked but only to type via the keys 0 to 9 and a to j. To program it, you need to install closed source software from an obscure Chinese website. So Peter started looking for other possibilities to program the keys of the mini-keyboard.