Port away from deprecated units API

Use Kirigami.Units instead of a global object.

discover/qml/AddSourceDialog.qml

@@ -50,7 +50,7 @@ Kirigami.OverlaySheet
 
             // Cancel out built-in margins so it lines up with the rest of the
             // content in this sheet
-            Layout.margins: -units.smallSpacing
+            Layout.margins: -Kirigami.Units.smallSpacing
 
             Button {
                 id: okButton

discover/qml/ApplicationsListPage.qml

@@ -146,7 +146,7 @@ DiscoverPage {
             height: visible ? Layout.preferredHeight : 0
 
             Item {
-                Layout.preferredHeight: units.gridUnit
+                Layout.preferredHeight: Kirigami.Units.gridUnit
             }
             Kirigami.Heading {
                 level: 2