Allow decorator properties to be repeated for multiple values for certain CSS properties

As I have I’ve previously [mentioned](http://qooxdoo.678.n2.nabble.com/Qooxdoo-3-0-decorator-questions-td7584521.html), I’ve patched MLinearBackgroundGradient and MLinearBackgroundGradient to allow for multiple background images and MBoxShadow to allow for multiple shadow definitions.

I’ve run the framework test suite of qx.test.ui namespace with the changes with no failures (but some test runs did have 133 failures but I did not get consistent results) though I’ve not added any new test cases for the new features.