Improve Tasks tree view in trunk

project.task
------------

seq, summary, project, user_id, deadline, remaining, progress, status, buttons

Note:
- Buttons is the following buttons, with small icons (no text):
»···. Start, Delegate, Done, Cancel
Only these 4 buttons must be displayed, with small icons -> small width of columns

project.task (with module project_gtd installed)
------------

- Add a few columns
seq, summary, project, timebox, button12, user_id, deadline, remaining, progress, status, buttons

Note:
- Button12:
»···. Up : move one timebox up
»···. Down : move one timebox down

Test if GTK client support putting small icons in buttons in tree views.