applets/fifteenpuzzle: hide board instead of covering it when solved
Fifteen Puzzle places a Rectangle over the board when the game is solved to hide the pieces and display the victory message (and full image if applicable). This doesn't look nice when put on the desktop with no image, as the rectangle uses the Theme background color, rather than the slightly transparent desktop widget color. It also leads to janky resizing behavior when solved, as the covered pieces may peek out from behind the rectangle as they are dynamically resized.
Instead, just hide the whole board when it is solved, and replace it with a victory sceen that does not force a background.
| before | after |
|---|---|
 |
 |