feat(ui5-cb-group-item): introduce nested grouping (draft)

[ndeshev]

Replace flat item structure with nested grouping.

Removes the ui5-cb-group-item component.

BREAKING CHANGE: The ui5-cb-group-item component is removed. Groups can now be created with the ui5-cb-item-group. Instead of using the old ui5-cb-group-item as separator in a flat structure:

<ui5-cb-group-item text="A"></ui5-cb-group-item>
<ui5-cb-item text="Algeria"></ui5-cb-item>
<ui5-cb-item text="Argentina"></ui5-cb-item>
<ui5-cb-item text="Australia"></ui5-cb-item>
<ui5-cb-item text="Austria"></ui5-cb-item>

The API supports nesting of ui5-cb-item components inside an ui5-cb-item-group:

<ui5-cb-item-group text="A">
	<ui5-cb-item text="Algeria"></ui5-cb-item>
	<ui5-cb-item text="Argentina"></ui5-cb-item>
	<ui5-cb-item text="Australia"></ui5-cb-item>
	<ui5-cb-item text="Austria"></ui5-cb-item>
</ui5-cb-item-group>