Overhaul planner-tasks-overview.

Overhaul planner-tasks-overview.

* planner-tasks-overview.el (planner-tasks-overview): Use
planner-extract-tasks.
(planner-tasks-overview-insert): Expect list layout returned by
planner-extract-tasks, and use parsing functions. Improve display.
Set working directory so that links work. Don't switch to
planner-mode, stay in planner-tasks-overview-mode.
(planner-tasks-overview-extract-all-tasks): Deleted because it's
redundant; we use planner-extract-tasks for this.
(planner-tasks-overview-mode): Derive from planner-mode, not
fundamental-mode.
(planner-tasks-overview-sort-by-date): Fix field number.
(planner-tasks-overview-sort-by-plan): Fix field number.
(planner-tasks-overview-sort-by-priority): Fix field number.
(planner-tasks-overview-sort-by-field): Handle nil properly for
sorting.
(planner-tasks-overview-sort-by-status): Fix field numbers.

git-archimport-id: mwolson@gnu.org--2006-planner-el/planner-el--devel--0--patch-135