Designer creating unsupported code for color palettes

Creating a color palette within designer creates a color for the QPalette::PlaceholderText role, even when this not supported in the selected Qt 5.9 kit (QPalette::PlaceholderText has been added with Qt 5.12).

Normally, the value is not generated unless actively modified, but the "Quick Tune Palette" button creates values for all roles.

This creates invalid code in the moc'd C++ file, requiring to edit the .ui file by hand (which is actively discouraged by Creator).

"Add a QPalette color role for placeholder texts" ebd3a13b807c6af2684b42d3912549caf7ef82aa https://codereview.qt-project.org/#/c/221775/