Fix margins for OnboardBanner

Re-usable components shouldn't really be setting layout themselves.
This should be done by whomever creates them.

The actual fix is that the banner no longe sets all margins, only left & right

CURA-8726

plugins/Marketplace/resources/qml/Marketplace.qml

@@ -92,6 +92,10 @@ Window
                     icon: content.item && content.item.bannerIcon
                     onRemove: content.item && content.item.onRemoveBanner
                     readMoreUrl: content.item && content.item.bannerReadMoreUrl
+
+                    Layout.fillWidth: true
+                    Layout.leftMargin: UM.Theme.getSize("default_margin").width
+                    Layout.rightMargin: UM.Theme.getSize("default_margin").width
                 }
 
                 // Search & Top-Level Tabs

plugins/Marketplace/resources/qml/OnboardBanner.qml

@@ -16,10 +16,7 @@ Rectangle
     property var onRemove
     property string readMoreUrl
 
-    Layout.preferredHeight: childrenRect.height + 2 * UM.Theme.getSize("default_margin").height
-    Layout.fillWidth: true
-    Layout.margins: UM.Theme.getSize("default_margin").width
-
+    implicitHeight: childrenRect.height + 2 * UM.Theme.getSize("default_margin").height
     color: UM.Theme.getColor("action_panel_secondary")
 
     // Icon