Use only one layout file. Following precedence is applied:

1. Local (i.e. $PWD/topydo_columns.ini or $PWD/topydo_columns.conf or
   $PWD/.topydo_columns)
2. User's home (i.e. $HOME/.topydo_columns or
   $HOME/.config/topydo/columns)
3. System-wide (i.e. /etc/topydo_columns.conf)

Gets rid of creation date and hidden tags that consume precious space.

Temporarily disable color output in column-ui's console

We don't want raw ANSI codes there. We'll enable it back again after
providing proper coloring solution.

New key-shortcuts for column-ui

Thanks to @bram85 for the idea.

Fix crashes when last item in column disappeared

Almost identical to `p`:
- `ps3d` will call `topydo postpone -s TODO 3d`
- `psy` will call `topydo postpone -s TODO 1y`

_command_on_selected: accepts string containing command (with potential
arguments) to execute and replace field (for further substitution with
todo id) as an argument.

There was uncaught IndexError when dealing with focus.

Actually it's a two letter shortcut. Second letter stands obviously for
priority.

Example:
- 'rc' will trigger `topydo pri TODO_ITEM C`

Shortcut calls postpone command and accepts similiar format as original
`topydo postpone`.

Examples:
- pressing 'pd' or 'p1d' will postpone selected task by one day.
- 'p3y' will postpone task by 3 years.
- 'p12w' will postpone task by 12 (twelve) weeks.

Introduce config file for column layout

Column-ui will look for columns configuration in:

1. system-wide configuration:
/etc/topydo_columns.conf

2. user dir:
~/.topydo_columns
~/.config/topydo/columns

3. current dir:
.topydo_columns
topydo_columns.conf
topydo_columns.ini

Each column should have its own INI style config section:

[FooBar]
title = Foo
filterexpr = Bar
sortexpr = desc:prio
show_all = 1

If any column option is missing, default value will be used.

Also move home_config_path() out of the Config class. It didn't use
'self' argument and this way we can reuse it other places.

Support backups (and `revert`) in column ui

Thanks to @mruwek for investigating.

Add `topydo columns` subcommand to launch column-ui

Switch column-ui to 256 color-mode

Drop python2 in column-ui and fix pylint errors

Fixes "not-callable" pylint errors.

A list of strings should be passed instead of a string.

Thanks to @mruwek for reporting.