IgniteUI / igniteui-webcomponents / 32125628778

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fix(dropdown): repair target lifecycle, selection state and ARIA (#2338)

Fixes:
- Enter on an open dropdown before any arrow key threw a TypeError.
- Keyboard navigation died once a dropdown opened at an explicit target was
  closed and reopened without that target, or after being moved in the DOM.
  The resolved target now outlives the open state.
- Reopening through the `target` slot left the list positioned at a previously
  passed, detached element.
- `show(other)` while open left `aria-expanded` on the previous element and
  leaked its key event listeners; `show('unknown-id')` threw.
- `clearSelection()` left navigation on the cleared item.
- `selectedItem` kept returning items removed from the DOM.
- `aria-haspopup` / `aria-expanded` outlived the dropdown on its target.
- `igc-dropdown-header` had no role inside the `listbox`, and an
  `igc-dropdown-group` label named nothing - both ports of what
  `igc-select-header` and `igc-select-group` already do.

Accessibility: the target declares `aria-haspopup="listbox"` and, while open,
`aria-activedescendant` for the item navigation is on, so the highlight is
announced. Focus stays on the target, which is what the key bindings are
observed on, so that is where the relation belongs.

Behavioral change: `igcChange` is no longer emitted when the already selected
item is picked again; the list still closes.

Refactor: `@watch` gives way to `willUpdate`, target resolution funnels through
one step that carries key listeners, outside-click exemption and ARIA with it,
selection follows select's shape, and the internals are `_`-prefixed and laid
out in regions.

16 new specs cover the fixed behaviors. No public API changes.

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