shell: Display search results for multiple query words

Before, queries like "remote desktop" were going through g_strstrip() and turning alternative matches to "remote" or "desktop" unreachable.

See #2763

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We still need to do a better job at adding keywords to the panels, as suggested in the linked issue.

I'm also working on making panel subpages reachable by search .

P.S.: These changes are small enough that they could go in gnome-45 too.