The margin around the icon didn't work if the icon source was none, because it was provided a QSize rather than a real.
The margin around the background didn't work at all because the background is automatically adjusted to fill the parent element, so it's best left out.
Contributes to issue CURA-8009.
This adds an option to the TextField element to show an icon on the left side in the text field. The icon remains visible when the user types in something (it's not part of the placeholder string).
Then we use that optional icon for the search bar in the Digital Library.
Contributes to issue CURA-8009.
