refactor(ui5-multi-combobox): enable hierarchical groups

[ivoplashkov]

Removes the ui5-mcb-group-item component.

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

<ui5-multi-combobox placeholder="Select a country">
	<ui5-mcb-group-item text="Asia"></ui5-mcb-group-item>
        <ui5-mcb-item text="Afghanistan"></ui5-mcb-item>
        <ui5-mcb-item text="China"></ui5-mcb-item>
        <ui5-mcb-item text="India"></ui5-mcb-item>
        <ui5-mcb-item text="Indonesia"></ui5-mcb-item>
	<ui5-mcb-group-item text="Europe"></ui5-mcb-group-item>
        <ui5-mcb-item text="Austria"></ui5-mcb-item>
        <ui5-mcb-item text="Bulgaria"></ui5-mcb-item>
        <ui5-mcb-item text="Germany"></ui5-mcb-item>
        <ui5-mcb-item text="Italy"></ui5-mcb-item>
</ui5-multi-combobox>

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

<ui5-multi-combobox placeholder="Select a country">
	<ui5-mcb-group text="Asia">
		<ui5-mcb-item text="Afghanistan"></ui5-mcb-item>
		<ui5-mcb-item text="China"></ui5-mcb-item>
		<ui5-mcb-item text="India"></ui5-mcb-item>
		<ui5-mcb-item text="Indonesia"></ui5-mcb-item>
	</ui5-mcb-group>
	<ui5-mcb-group text="Europe">
		<ui5-mcb-item text="Austria"></ui5-mcb-item>
		<ui5-mcb-item text="Bulgaria"></ui5-mcb-item>
		<ui5-mcb-item text="Germany"></ui5-mcb-item>
		<ui5-mcb-item text="Italy"></ui5-mcb-item>
	</ui5-mcb-group>
</ui5-multi-combobox>

Relates to #8461