applets/fifteenpuzzle: make tiny sizes work better
We allow users to make the applet really small, which can leave it unplayable in some configurations. Part of that is a clear "Don't do that then" case (if you want to play on a 18×18 grid, don't make it barely larger than a stamp) but we can do a much better job at making it fit on the smallest sizes.
This change enables word wrap for the victory screen labels, replaces the shuffle button with an icon-only one if its row would not fit, and fixes a spurious binding (my bad) that can cause the field to escape the plasmoid size if you resize it in some ways.
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