revert #680 from xxMUROxx/hotfix/FontSizeProperty-TabItem

Removed HeaderFontSize-DP and used FontSize-DP in TabItem
need this for #739  (reverted from commit 10326e7f480155bfd9903a4cff31d69612d89814)

this breaks the inner FontSize!!!
revert #680 from xxMUROxx/hotfix/FontSizeProperty-TabItem

Removed HeaderFontSize-DP and used FontSize-DP in TabItem
need this for #739  (reverted from commit 10326e7f480155bfd9903a4cff31d69612d89814)

this breaks the inner FontSize!!!