Draft: fix wrong type for layer transparency

[Roy-043]

Transparency should be "App::PropertyPercent"
Also fixed a typo.

Forum post: https://forum.freecadweb.org/viewtopic.php?f=23&t=60893

Did not add the migrate function proposed on the forum. Seems overkill.

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[yorikvanhavre]

Typo indeed I think. Thanks!