Keep showing search icon while typing search query

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.

resources/qml/Widgets/TextField.qml

@@ -15,6 +15,8 @@ TextField
 {
     id: textField
 
+    property alias leftIcon: iconLeft.source
+
     UM.I18nCatalog { id: catalog; name: "cura" }
 
     hoverEnabled: true
@@ -22,6 +24,7 @@ TextField
     font: UM.Theme.getFont("default")
     color: UM.Theme.getColor("text")
     renderType: Text.NativeRendering
+    leftPadding: iconLeft.visible ? iconLeft.width + UM.Theme.getSize("default_margin").width * 2 : UM.Theme.getSize("thin_margin")
 
     states: [
         State
@@ -67,5 +70,23 @@ TextField
             }
             return UM.Theme.getColor("setting_control_border")
         }
+
+        //Optional icon added on the left hand side.
+        UM.RecolorImage
+        {
+            id: iconLeft
+
+            anchors
+            {
+                verticalCenter: parent.verticalCenter
+                left: parent.left
+                leftMargin: UM.Theme.getSize("default_margin").width
+            }
+
+            visible: source != ""
+            height: UM.Theme.getSize("small_button_icon").height
+            width: visible ? height : 0
+            color: textField.color
+        }
     }
 }