feat(ui5-combobox): add suggestions grouping

[ndeshev]

Enable users to set items to groups. Groups are visually identified in the suggestions list by group headers.

For group headers a new component is introduced - ComboBoxGroupItem. The grouping is based on the items order as they are declared in the markup - the items between two group headers are considered to belong to the first one.

Example - in the code below - the items between the "A" group item and the "Random Group Item" are grouped to the "A" group while the last item belongs to the "Random Group Item Text" group.

        <ui5-combobox id="combo-grouping">
		<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>

		<ui5-cb-group-item text="Random Group Item Text"></ui5-cb-group-item>
		<ui5-cb-item text="Albania"></ui5-cb-item>
	</ui5-combobox>

Fixes: #3371

[fifoosid]

Add ui5-cb-group-item to the appenddocs tag in ComboBox.js
Add a sample in ComboBox.sample.html

[MapTo0]

I suggest we deep test this changed once the value change pull request get merged

[fifoosid]

LGTM now, someone from @SAP/ui5-webcomponents-topic-rl should approve and we can merge it

[MapTo0]

/ ---------- /

1. type "b"
2. arrow down - b should be removed

/ ---------- /

1. Type "a"
2. items gets filtered, but not autocompleted

[MapTo0]

Overall. looks good, fix these issue and we should be good to go.

[MapTo0]

Actually, there is a double focus all the time, but I guess this should be another issue ..

[ndeshev]

There is no double focus for me? Please pull the last changes, something is not right.
When a group item is reached with the keyboard the typed in value should not be cleared, as it is in UI5.

going on a vacation

[hristop]

lgtm, I think we can merge when someone with rights approve the change