Implement OPML import in the desktop and touch ui

The core has already implemented parsing OPML files from
URLs and files, it only needs to be exposed by the UI which
this parch does.

desktop/dialogs/AddPodcastDialog.qml

@@ -7,6 +7,7 @@ import '../common'
 import '../common/util.js' as Util
 
 Dialog {
+    property alias labelText: urlEntyLabel.text
     signal addUrl(string url)
 
     width: 300
@@ -18,6 +19,7 @@ Dialog {
         anchors.fill: parent
 
         Label {
+            id: urlEntyLabel
             text: 'URL:'
         }