libinputkeyboard should have a way to change repeat delay and rate

Currently, qlibinputkeyboard.cpp has the repeat delay and rate hard-coded:

const int REPEAT_DELAY = 500;
const int REPEAT_RATE = 100; 

It would be nice to have a way to change them.

A quick and dirty solution that could work would be to copy what is done in qevdevkeyboardmanager.cpp and read the delay and rate from an environment variable.