Update alttab to version 1.6.0 / rev 5 via SR 879121
    
    https://build.opensuse.org/request/show/879121
    by user jubalh + dimstar_suse
    - Update to 1.6.0:
      * Fix major memory leak when huge icons are not used.
      * Debug by SIGUSR1.
      * Add test suite: running in headless X server.
      * Add github CI pipeline.
      * Bspwm documentation.
      * Fix possible strncpy overflows.