misc: Add Luke-jr back to AUTHORS.md and reorder a bit.
core: Accidentally left a part of a previously non-merged commit (thread display).

IMO threads don't change dynamically, so they shouldn't be displayed
in the main window, but in "GPU settings" menu instead.
core: initial implementation of user-settable xintensity. Has a bug.

Changing `intensity` and to it from `xintensity` works fine.

Changing `xintensity` fails to enqueue kernel immediately:

    [04:13:01] Error -54: Enqueueing kernel onto command queue. (clEnqueueNDRangeKernel)
    [04:13:01] GPU 0 failure, disabling!

Tests show that manually enabling this disabled GPU works fine.