Selection lost when closing autocomplete without selecting option

Problem/Motivation

If the search/autocomplete overlay is closed without selecting an option (e.g. when clicking somewhere else) the menu tree gets closed, the search field gets cleared and if a menu item's parent was selected, this selection is lost.

Steps to reproduce

Edit or create a node. Start creating a menu item. Optionally select a menu item's parent in the tree. Use the search field:
1. Type a string that exists in a page title. When the options are displayed click somewhere else.
or
2. Type a string that doesn't exist in a page title.

Proposed resolution

When listening to the 'autocompleteclose' event, check whether the value is a existing link key.

Remaining tasks

User interface changes

API changes

Data model changes