Use special color-palette entry for 'marked' items

This improves readability. Marked items look now differently from the
focused one.

topydo/ui/Main.py

@@ -179,6 +179,7 @@ class UIApplication(CLIApplicationBase):
             ('link', '', '', '', link_color, ''),
             ('link_focus', '', 'light gray', '', link_color, None),
             ('default_focus', 'black', 'light gray'),
+            ('marked', '', 'light blue'),
         ]
 
         for C in ascii_uppercase:

topydo/ui/TodoWidget.py

@@ -135,7 +135,14 @@ class TodoWidget(urwid.WidgetWrap):
         return True
 
     def mark(self):
-        self.widget.set_attr_map({None: _markup(self.todo, True)})
+        attr_map = {
+            None:       'marked',
+            'link':     'marked',
+            'context':  'marked',
+            'project':  'marked',
+            'metadata': 'marked',
+        }
+        self.widget.set_attr_map(attr_map)
 
     def unmark(self):
-        self.widget.set_attr_map({None: _markup(self.todo, False)})
+        self.widget.set_attr_map(_markup(self.todo, False))