For users with multiple keyboard layouts configured, glfwGetKeyName
works fine only with the primary layout.  Switching layouts results in
changing the group index.  This commit querries the current group index
when initializing keyboard input and keeps track of any change to it.

As a result the scancode -> keyname mapping may change while the program
is running (needs to be documented).

Fixes #1462.
Closes #1528.

(cherry picked from commit 36f90800d8d490b27d4dc2109c07a95a12007327)