Segfault from NSMatrix

[Fred Kiefer]

The last changes on NSMatrix have broken the operation of the NSBox test
program in GSTest. I tried to fix it myself ([self sendAction: _action
to: _target]; instead of   [_target performSelector: _action withObject:
self]; in _mouseDownNonListMode:) but this does not solve the whole
problem. So perhaps someone with more insight could have a look at what
should happen here. 

And please remember to run the test programs before adding code to the
CVS.

Fred