Field is still in edit mode after being blurred by tab navigation

This issue is introduced by PR #1338 (see [1]) that introduces tab navigation in the metadata property sheet. Its cause is that the library angular-chosen does not yet support the events ng-blur which would be required to switch of the edit mode when losing the keyboard focus.

See https://github.com/leocaseiro/angular-chosen/issues/98

Steps to reproduce:
1. Open "Add Event"
2. Use "Tab" to navigate through the fields

Actual Results:
When a selection field (containing a selection dropdown list) loses the keyboard focus, it is still in the edit mode.

Expected Results:
When a selection field loses the keyboard focus, the edit mode should be disabled.

Workaround (if any):
Just ignore it - it works.

[1] https://bitbucket.org/opencast-community/matterhorn/pull-requests/1338/t-mh-11849-improve-keyboard-accessibility/diff