Fixes bug where qx.data.Array.splice fires wrong/inaccurate change event

This pull request was previously released as pull request #19 but was marked as already fixed - it's not (I think I screwed up my original commit); the problem can be seen here [1] and is logged here [2].

This branch and pull request fixes the problem, including unit tests

[1] http://tinyurl.com/d826eyj 
[2] http://bugzilla.qooxdoo.org/show_bug.cgi?id=6207