Task editor subtitle to indicate task creation date and number of blocked parents

[nekohayo]

I sometimes could benefit from realizing how long ago a task has been opened/created, and how many other tasks it blocks. GTG has this information (creation date) in its data file format but doesn't expose it anywhere. As for the blocked tasks, there probably is an easy way in GTG / liblarch to query how many levels of parent tasks are blocked by the currently displayed task, though that may be something that ought to wait until #737 / #553 is done.

For displaying the information, it might be simply using a text underneath the title text in the task editor's headerbar, using the has-subtitle and subtitle properties of the GTK HeaderBar widget, or the set_has_subtitle & set_subtitle functions. The text could say something like this:

Created {{int number}} days ago, blocks {{number}} tasks

Created {{float number}} months ago, blocks {{number}} tasks

Created {{float number}} years ago, blocks {{number}} tasks

(with the "blocks X tasks" part being optional/generated only if necessary, and using ngettext for correct translation-friendly plurals, of course)

I'm not sure if this sort of feature is plugin material or a core feature, and the potential performance implications, so tagging this ticket as "plugins" just in case.