Change Con.mark to marks

In newer versions of i3 containers can have multiple marks, so the property
name has changed form mark to marks.

i3ipc.py

@@ -404,7 +404,7 @@ def __init__(self, data, parent, conn):
 
         # set simple properties
         ipc_properties = ['border', 'current_border_width', 'focused',
-                          'fullscreen_mode', 'id', 'layout', 'mark', 'name',
+                          'fullscreen_mode', 'id', 'layout', 'mark', 'marks', 'name',
                           'orientation', 'percent', 'type', 'urgent', 'window',
                           'num', 'scratchpad_state']
         for attr in ipc_properties:
@@ -413,6 +413,15 @@ def __init__(self, data, parent, conn):
             else:
                 setattr(self, attr, None)
 
+        # Make sure marks is never None (use []) to simplify client code. Older
+        # versions of i3 used mark instead of marks, so collapse that into
+        # marks to provide a consistent API.
+        if not self.marks:
+            self.marks = []
+            if self.mark:
+                self.marks.append(mark)
+            del self.mark
+
         # XXX this is for compatability with 4.8
         if isinstance(self.type, int):
             if self.type == 0:
@@ -540,8 +549,9 @@ def find_classed(self, pattern):
                 if c.window_class and re.search(pattern, c.window_class)]
 
     def find_marked(self, pattern=".*"):
+        pattern = re.compile(pattern)
         return [c for c in self.descendents()
-                if c.mark and re.search(pattern, c.mark)]
+                if any(pattern.search(mark) for mark in c.marks)]
 
     def find_fullscreen(self):
         return [c for c in self.descendents()