feat: 适配社区版5.15环境下调色板问题

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 b27f489)

src/kernel/dguiapplicationhelper.cpp

@@ -540,6 +540,13 @@ DPalette DGuiApplicationHelper::standardPalette(DGuiApplicationHelper::ColorType
         QPalette::ColorRole role = static_cast<QPalette::ColorRole>(i);
         QColor color = qcolor_list[i];
 
+#if QT_VERSION >= QT_VERSION_CHECK(5, 12, 0)
+        if (role == QPalette::PlaceholderText) {
+            // 5.15新添加此颜色 这里使用5.11的颜色保证效果与5.11对齐
+            color = dcolor_list[DPalette::PlaceholderText];
+            continue;
+        }
+#endif
         // 处理半透明色
         if (allowCompositingColor) {
             switch (role) {