Idea: show logbook entries

Idea from Alexander Baier:

https://lists.gnu.org/archive/html/emacs-orgmode/2017-07/msg00448.html

> Is it possible to alter the text shown for each headline after toggling entry-text-mode in the agenda? At the moment, a certain number of lines of the headline's body is displayed. Instead of the lines I would like to display the last note that has been recorded in a LOGBOOK drawer.

> My scenario: I have an agenda that lists stuck headlines (no child headline is TODO and at least one is BLOCKED). I record a note when transitioning to the BLOCKED state which gives a reason for the blocking. I want to collect the reasons of all the blocked child headlines and display them under the parent headline in the agenda view (when in entry-text-mode).