Change all the occurrences of `(opens in a new window)` to `(opens in a new tab)`

[afercia]

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.

Considering also the propsed change in Gutenberg, see ​https://github.com/WordPress/gutenberg/pull/6245, I'd propose to just change "window" to "tab".

[chetan200891]

Created inital patch to change text from "window" to "tab".

[chetan200891]

Added initial patch to change text from "opens in a new window" to "opens in a new tab".

[ianbelanger]

I had a little trouble applying the patch so I created one with the same changes.

[webzunft]

Checked the patch (43803.2.diff) and it does what it was supposed to. No other occurrences of "opens in a new window" found.

[SergeyBiryukov]

In 43174:

Administration: Change all the occurrences of "(opens in a new window)" to "(opens in a new tab)".

Props chetan200891, ianbelanger, afercia.
Fixes #43803.

[audrasjb]

Reopening for 4.9.9 consideration as seen during accessibility team bug-scrub meeting.

[pento]

This is fine to be released in 5.1.

[afercia]

In 44655:

Build/Test Tools: Update QUnit index file after [43174].

Updates the last occurrence of "(opens in a new window)" to "(opens in a new tab)".

See #43803.