Accessibility updates based on jQuery UI Autocomplete

Instead of rewriting the labels it creates a new offscreen label. Rewriting the original label leaves the original control without a label and as a result it fails accessibility tests.

Added offscreen label for focusser.

Added offscreen status element acting as a aria live region. This is based on the implementation present in jQuery UI Autocomplete. The enables readers to speak the select2 instances status as it's updated. Either by search to navigating the results with the keyboard.

Adds new locale item `formatMatches`.