ENH: Added the ability of Reference Items to have Active Children

Example SBI Syntax:

        <Component Name="material" Label="material" Version="0" EnforceCategories="false" Role="-2" NumberOfRequiredValues="1">
          <CategoryInfo Inherit="true" Combination="All" />
          <Accepts>
            <Resource Name="smtk::attribute::Resource" Filter="attribute[type='Material']" />
          </Accepts>
          <Rejects />
          <ChildrenDefinitions>
            <Double Name="initialFlow" Label="initialFlow" Version="0" NumberOfRequiredValues="3">
              <CategoryInfo Inherit="true" Combination="All" />
            </Double>
            <Double Name="initialTemp" Label="initialTemp" Version="0" NumberOfRequiredValues="1">
              <CategoryInfo Inherit="true" Combination="All" />
            </Double>
          </ChildrenDefinitions>
          <ConditionalInfo>
            <Condition Resource="smtk::attribute::Resource" Component="attribute[type='SolidMaterial']">
              <Items>
                <Item>initialTemp</Item>
              </Items>
            </Condition>
            <Condition Component="attribute[type='LiquidMaterial']">
              <Items>
                <Item>initialTemp</Item>
                <Item>initialFlow</Item>
              </Items>
            </Condition>
          </ConditionalInfo>
        </Component>

qtReferenceItemComboBox has been renamed to qtReferenceItemEditor to reflect the fact that the class now supports the active children of ReferenceItems

Also fixed resizing issue with the attribute definition combobox in qtAttributeView

Closes #408 (closed)