webadmin: EnterIgnoringFocusHandler catch ONKEYDOWN

EnterIgnoringFocusHandler -> onPreviewNativeEvent:
execute 'enterPressed' on ONKEYDOWN event instead of
ONKEYPRESS to overcome an issue in newer Chrome/FF
browsers, in which the textarea doesn't catch the
ONKEYPRESS event. Consequently, pressing enter key
in a textarea won't break to a new line
(e.g. 'Custom Script' textarea).

Change-Id: I064f74e79dedb20ab305ef238c7ff2a6fed743fe
Bug-Url: https://bugzilla.redhat.com/1064567
Signed-off-by: Daniel Erez <derez@redhat.com>