Decrease size of the icons in the marketplace onboarding banners

CURA-8564

plugins/Marketplace/resources/qml/OnboardBanner.qml

@@ -36,8 +36,8 @@ Rectangle
             left: parent.left
             margins: UM.Theme.getSize("default_margin").width
         }
-        width: UM.Theme.getSize("button_icon").width
-        height: UM.Theme.getSize("button_icon").height
+        width: UM.Theme.getSize("banner_icon_size").width
+        height: UM.Theme.getSize("banner_icon_size").height
     }
 
     // Close button

resources/themes/cura-light/theme.json

@@ -684,6 +684,8 @@
         "table_row": [2.0, 2.0],
 
         "welcome_wizard_content_image_big": [18, 15],
-        "welcome_wizard_cloud_content_image": [4, 4]
+        "welcome_wizard_cloud_content_image": [4, 4],
+
+        "banner_icon_size": [2.0, 2.0]
     }
 }