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.

touch/PodcastsPage.qml

@@ -66,6 +66,20 @@ SlidePage {
                     });
                 },
             },
+            {
+                label: 'Add from OPML',
+                callback: function () {
+                    var ctx = { py: py };
+                    pgst.loadPage('TextInputDialog.qml', {
+                        buttonText: 'Subscribe',
+                        placeholderText: 'OPML URL',
+                        pasteOnLoad: true,
+                        callback: function (url) {
+                            ctx.py.call('main.import_opml', [url]);
+                        }
+                    });
+                },
+            },
             {
                 label: 'Discover new podcasts',
                 callback: function () {