move icons to xdg standard dirs, add desktop files

closes #359

weboob/core/backends.py

@@ -24,6 +24,7 @@ from logging import debug, error, exception, warning
 import os
 import re
 import stat
+import xdg.IconTheme
 
 from weboob.capabilities.cap import ICap
 from weboob.tools.backend import BaseBackend
@@ -69,6 +70,8 @@ class Backend(object):
 
     @property
     def icon_path(self):
+        if self.klass.ICON is None:
+            self.klass.ICON = xdg.IconTheme.getIconPath(self.klass.NAME)
         return self.klass.ICON
 
     def iter_caps(self):