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. This enables readers to speak the select2 instance's status as it's updated. When someone enters a search term or when they navigate the results with the keyboard.

Adds new locale item `formatMatches`.

Tested using WAVE and VoiceOver.