Specify spacing between search bar and button

Instead of using the default of 5 pixels, we should use this element from the theme to match the margin around the page.

Contributes to issue CURA-8009.

plugins/DigitalLibrary/resources/qml/SelectProjectPage.qml

@@ -42,6 +42,7 @@ Item
             right: parent.right
         }
         height: childrenRect.height
+        spacing: UM.Theme.getSize("default_margin").width
 
         Cura.TextField
         {