Allow setting colorkey if possible. Implement property probe interface for optional X features (autopaint-colorkey, d...

Original commit message from CVS:
* sys/xvimage/xvimagesink.c:
* sys/xvimage/xvimagesink.h:
* tests/icles/Makefile.am:
* tests/icles/test-colorkey.c:
Allow setting colorkey if possible. Implement property probe interface
for optional X features (autopaint-colorkey, double-buffer and
colorkey). Fixes #554533