darwin: Fix uiTable checkbox change not calling SetCellValue().

[szanni]

This is something that came up while trying things out with the new tester for uiTable suggested in #508:

The SetCellValue() when clicking a checkbox within a uiTable is never called. The UI happiliy updates suggesting the value has been set - which is not actually the case.

The checkbox not redrawing with the actual check box value from the model is another related issue, see #507.

As a general question regarding PRs fixing issues: should I open a separate issue every time? Or is the PR fixing the issue enough? I couldn't find anything in CONTRIBUTING.md