[applets/taskmanager] Change "open new instance" to "open new window"

"Instance" is an esoteric jargon word. Now, technically a new instance of an
app and a new window of the same app are different things, but to most users,
it's a distinction without a difference, and the users who do understand the
difference won't be confused by the use of more user-friendly terminology
2 jobs for !133 with open-new-window-text in 2 minutes and 49 seconds (queued for 59 seconds)
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