Added GroupTitleStyle to allow styling of propertyGroups

[nergmada]

So whilst trying to use the RadDataForm I couldn't seem to style Group titles for property groups. This PR basically just registers the GroupTitleStyle object so that it can be used to style groups as shown in example below

<propertyGroup bind:this={group} collapsible="true" name="Additional Options" hidden="false">
  <propertyGroup.titleStyle>
    <groupTitleStyle fillColor="#ffffff" />
  </propertyGroup.titleStyle>
  <entityProperty name="marks" displayName="Marks" index="3">
    <propertyEditor type="Numeric" />
  </entityProperty>
</propertyGroup>

Edit: For reference, here's a link to the NativeScript documentation regarding this