window: Prevent creating new views if already loaded

Evan Nehring requested to merge enehring/gnome-music:dupe-views into master

Closes: #387 (closed)

This is a simplistic fix to the issue. @mschraal mentioned the code could use a rewrite, but I'm not sure how much effort is required there, so I thought this could be used as a temporary fix until then.

If work is already underway for a rewrite, feel free to ignore this MR.

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