The function to generate a list of GrepFilters, OrdinalTagFilters
NegationFilters is needed by the new UI. Move this functionality
the ExpressionCommand to the Filter module.

This is to fix some pylint errors. Pylint couldn't reason about the
urwid classes because they were not installed.

This is implemented using the ListBox widget.

Output is still shown for commands entered on the commandline.

Use the emit_signal and register_signal methods to properly set the
signals that CommandLineWidget can emit.

Started on the UI that I envisioned since the very beginning of this
project, 364 days ago.

The UI is inspired by TweetDeck, the column-like Twitter interface.
Every column is associated with a View object (which has a filter and a
sort order on the active todo.txt file). So each column can show a
particular part of the todo.txt files. E.g, a column for today's items,
a column for overdue items or a column for a particular project
(filter: GrepFilter("+SomeProject")).

The way topydo is designed was always with this UI in mind. Commands
('add', 'do', etc.) should be independent of the UI that invoked them,
therefore they never print to stdout but output to a function such that
the current UI will show it properly. Also the View class, which hasn't
been incredibly useful sofar, can finally fulfil its role better.

This is still a very immature version of the UI. It shows a single
column with all the todos. It can execute commands, but it will do so
silently, no output or errors are shown yet.

The navigation will be Vim-like. Press : to toggle the command-line at
the bottom of the screen, where you can enter any command that you're
used to. Press Escape entering a command to set the focus back on the
columns.

The next few items to address:
* show output, errors and handle input
* ability to dynamically add/edit/delete columns
* add navigation keys to 'walk' through the items in the columns and act
  on them with shortcuts. E.g. highlight a todo, press 'x' to complete
  it.

I chose urwid as the widget library for the console, since using curses
directly gave me a headache.

When a subcommand finishes, _post_excecute() is called for sorting,
archiving and writing the file. But sorting is also a subcommand, which
in turn also calls _post_execute(), etc, etc.

So let the caller of _execute() decide when to call _post_execute.

This fixes issue #46.

Just make sure that proper arguments are passed to GrepFilter in the
first place.

This started as a refactoring of the View class: to get rid of the
pretty_print() function and anything releated to printers. So the View
class now solely focusses on sorting and viewing a bunch of todos. It's
up to the users of the View class to retrieve the todos in the view and
pass them on to a Printer instance.

The View class was quite involved with the proper Unicode support that
was added lately. There were still encoded string passed through this
class rather than Unicode strings. Getting rid of this affected
basically all other places in the codebase.

The result is that there are less string conversions; printed todo items
stay Unicode as long as possible, until they're actually written out to
standard output or to a file. I got rid of one text_type() call from the
six library, all @python_2_unicode_compatible decorators are gone and
the tests no longer need the utf8() function because everything is
Unicode now.

Remove utf8 function.

No need to toggle the highlighting of projects and contexts anymore. If
you want to turn it off, simply assign an empty value to the
project/context colors:

  [colorscheme]
  project_color =
  context_color =

Thanks to Jacek for pointing out.

Safe 16 base colors