org-agenda: Track last indirect buffer
* lisp/org-agenda.el (org-agenda-tree-to-indirect-buffer): Record last
indirect buffer for agenda.
* lisp/org-agenda.el (org-agenda-quit): Delete window for
org-agenda-last-indirect-buffer, not org-last-indirect-buffer.
Introduce a new variable, org-agenda-last-indirect-buffer, that is
specific for the agenda (and perhaps a particular agenda buffer, if
org-agenda-sticky is set). This prevents org-agenda-quit from deleting
a window if it is displaying a buffer that was created through a
org-tree-to-indirect-buffer call outside of
org-agenda-tree-to-indirect-buffer.
Make org-agenda-Quit check for indirect buffer windows, like
org-agenda-quit does.