Basis of header line

I think the icon on the button is too small, but that's not currently configurable. Will have to look into that.

Contributes to issue CURA-8565.

plugins/Marketplace/resources/qml/PackageDetails.qml

@@ -3,8 +3,45 @@
 
 import QtQuick 2.15
 import QtQuick.Controls 2.15
+import QtQuick.Layouts 1.3
 
-Label
+import Cura 1.0 as Cura
+import UM 1.0 as UM
+
+Item
 {
-    text: "Test!"
+    Column
+    {
+        anchors.fill: parent
+        anchors.margins: UM.Theme.getSize("default_margin").width
+
+        RowLayout
+        {
+            spacing: UM.Theme.getSize("default_margin").width
+
+            Cura.SecondaryButton
+            {
+                Layout.alignment: Qt.AlignVCenter
+                Layout.preferredHeight: UM.Theme.getSize("action_button").height
+                Layout.preferredWidth: height
+
+                onClicked: contextStack.pop() //Remove this page, returning to the main package list or whichever thing is beneath it.
+
+                tooltip: catalog.i18nc("@button:tooltip", "Back")
+                toolTipContentAlignment: Cura.ToolTip.ContentAlignment.AlignRight
+                iconSource: UM.Theme.getIcon("ArrowLeft")
+                leftPadding: UM.Theme.getSize("narrow_margin").width
+                rightPadding: leftPadding
+            }
+
+            Label
+            {
+                Layout.alignment: Qt.AlignVCenter
+
+                text: "Install Plug-ins" //TODO: Depend on package type, and translate.
+                font: UM.Theme.getFont("large")
+                color: UM.Theme.getColor("text")
+            }
+        }
+    }
 }