added setTristateBoolean/getTristateBoolean
new Composite TristateCheckbox (checkbox with
checked/unchecked/undefinde)
LabeledInputField, enhanced with CHECKBOX/CHECKBOXTRISTATE; adapted for
new class/Composite TristateCheckbox
Merge pull request #24 from marlovitsh/LabeledInputField_Enhancements

Labeled input field enhancements - new version as requested