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.

main.py

@@ -32,6 +32,7 @@ from gpodder.api import core
 from gpodder.api import util
 from gpodder.api import query
 from gpodder.api import registry
+from gpodder import opml
 
 import logging
 import functools
@@ -221,6 +222,21 @@ class gPotherSide:
         summary.sort(key=lambda e: e['newEpisodes'], reverse=True)
         return summary[:int(count)]
 
+    @run_in_background_thread
+    def import_opml(self, url):
+        """Import subscriptions from an OPML file
+
+        import http://example.com/subscriptions.opml
+
+            Import subscriptions from the given URL
+
+        import ./feeds.opml
+
+            Import subscriptions from a local file
+        """
+        for channel in opml.Importer(url).items:
+            self.subscribe(channel['url'])
+    
     @run_in_background_thread
     def subscribe(self, url):
         url = self.core.model.normalize_feed_url(url)
@@ -545,6 +561,7 @@ load_podcasts = gpotherside.load_podcasts
 load_episodes = gpotherside.load_episodes
 show_episode = gpotherside.show_episode
 play_episode = gpotherside.play_episode
+import_opml = gpotherside.import_opml
 subscribe = gpotherside.subscribe
 unsubscribe = gpotherside.unsubscribe
 check_for_episodes = gpotherside.check_for_episodes