Changed qtInputsItem so that child items appear to the right of the combobox

DiscreteValueEditors with children used to render with the child beneath the ComboBox:

After changing the rendering to display the children to the right of the ComboBox:

This work is meant to address SMTK Issue #417

Three test cases are present in this SBT file: ExtensibleSquished-3TestCases.sbt

1. The original test case - the ComboBox having one child
2. The ComboBox having more than one child (use the Foo option in the upper if panel)
3. The ComboBox being Extensible, but having no children (in the bottom if else panel)