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.
This is the standard style for text fields in Cura. The spacing around the search icon is customised with the best approximation I could get using a simple string of placeholderText.
Contributes to issue CURA-8009.
Seems to work fine.
The pyqtSignal is not technically necessary unless we display this filter in the interface anywhere. We don't currently. If the signal is not present it would complain about non-notifyable properties. And making it a slot instead of a signal seems a bit unintuitive to me in how the field is being used.
Contributes to issue CURA-8009.
