There is a Search field with the magnification-lens and a greyed out \"search\" keyword at the top right corner of stackoverflow.com web site:
Here's a way to achieve this with stylesheets only:
QLineEdit {
background: #f3f3f3;
background-image: url(:Images/search.svg); /* actual size, e.g. 16x16 */
background-repeat: no-repeat;
background-position: left;
color: #252424;
font-family: SegoeUI;
font-size: 12px;
padding: 2 2 2 20; /* left padding (last number) must be more than the icon's width */
}
Here's the result:
It's still not perfect. You don't have much influence over the icon's position.