Implements key repeats when a key is pressed and hold.
~~There is still an issue when multiple keys are pressed at the same time. At some point, the 'event' in 'GHOST_EventPrinter::processEvent' is NULL:
```
Thread 1 "blender" received signal SIGSEGV, Segmentation fault.
0x00000000059eb5d5 in GHOST_EventPrinter::processEvent (this=0x602000002c90,
event=0xbebebebebebebebe)
at /home/christian/Development/blender/intern/ghost/intern/GHOST_EventPrinter.cpp:39
39 if (event->getType() == GHOST_kEventWindowUpdate)
```
This appears to be a synchronisation issue.
Any ideas how the event can become NULL? They are very likely not NULL when they are added to the event queue by `pushEvent(new GHOST_EventKey(...))`.~~