SourcesPage: Use ItemIsUserCheckable to check if a source is checkable

We use it to disable the control if it's not, this way the backend can
support to tell the state but not allow interaction.


(cherry picked from commit 5365c43a)

discover/qml/SourcesPage.qml

@@ -210,7 +210,7 @@ DiscoverPage {
                     readonly property variant idx: sourcesView.model.index(index, 0)
                     readonly property variant modelChecked: sourcesView.model.data(idx, Qt.CheckStateRole)
                     checked: modelChecked !== Qt.Unchecked
-                    enabled: modelChecked !== undefined
+                    enabled: sourcesView.model.flags(idx) & Qt.ItemIsUserCheckable
                     onClicked: {
                         sourcesView.model.setData(idx, checkState, Qt.CheckStateRole)
                         checked = Qt.binding(function() { return modelChecked !== Qt.Unchecked; })