Change all the occurrences of new window to new tab

[afercia]

Worth considering in core all occurrences of (opens in a new window) have been changed to (opens in a new tab), see https://core.trac.wordpress.org/ticket/43803 which will be released in 5.1 (note: not in 5.0).

In Gutenberg, the occurrences of new window are sometimes used for visually hidden accessible text and sometimes as visible labels. Regardless, for consistency and preparation with the change landing in core, I'd propose to change all occurrences to new tab.

[afercia]

Also for clarification, quoting from the Trac ticket:

today, it's very likely that browsers will open links with a target="_blank" attribute in a new browser's tab rather than in a new window. It is true that this depends also on user settings, but in the vast majority of the cases they will open in a new tab.

[afercia]

Related: #9520 which is going to introduce a "new window" string.