feat(ui5-tabcontainer): make tab-select event preventable

[georgimkv]

The tab-select event can now be prevented.

Added new public getter allItems that returns an array which includes all slotted tabs and their subTabs. The reason for the addition is that the tab-select event returns a tabIndex of the selected tab, but if that selected tab is nested, then tabIndex will not correspond to the items property, which includes only the top-level tabs.

Fixes #5116