- Create a 'Help-Widget' to give the user an information about all possible
  keys.

- reimplement RPN input for current getan implementation (28489e672e61)

- Improve default colors, e.g. yellow on green for the current time is hard to
  read, and the currently selected project has a black background, which is
  not much of a highlight on a terminal with a black background.

- The sum of previous times and currently running time should be displayed as
  in previous versions of getan.

- The currently running time should be shown near the project.

- Allow empty descriptions.

- When adding/removing time, the highlighting of the current project is lost.