Fix #306832 - AppImage doesn't store the correct wallpaper filename f…

[njvdberg]

…or Background and Paper

Resolves: https://musescore.org/en/node/306832 (partially)

When for the foreground a wallpaper is selected but the file is not found, ScoreView falls back on the foreground color White. However, in ExampleView the foreground color _fgColor wasn't set so it falls back on the default color Black.
Now the default foreground color of ExampleView is also set to white.

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