qt5.12中新增了PlaceholderText颜色，添加与5.11
颜色一致的对应颜色到此调色板中。使风格保持一致

Log:
Bug: https://pms.uniontech.com/zentao/bug-view-66654.html
Change-Id: I247d1619b9ec24582c09945bb2b3f4cd913fd4f5
(cherry picked from commit b27f4898127133017c14b793c27f452958c7eff2)