Show cleaned-up extended description in a tooltip on hover

[nekohayo]

I have the disease of typically putting EVERYTHING in the task titles, and the reason I do often do that is that subconsciously, I know I won't read the task's extended description because:

1. I won't know/remember it's there, particularly if the title is long-enough that the description doesn't show on the right as grayed out text
2. It requires me to double-click to open a task, which not only is a conscious action with effort involved, it's also a ton of extra window management and I already have a gazillion windows open

Yet, I hesitated in suggesting the use of a visual indicator / special icon to indicate "this task has an extended description", so I've been stuck with writing "[notes]" in a lot of my task titles to remind me that there's "more important details inside the task".

Today, a potential solution just struck me: what if, only in the case of tasks that have an extended description (not just a title and tags), we used a GtkTooltip showing that description (without the title and tags) on hover in the tasks list in the main window? Boom, now we've got the ability to "peek" into a task!

[nekohayo]

Further ideas: this could also let you know, when you are in the "actionable" (flattened) view, if a task has parents or children. So in that case, the tooltip could look like this as two translatable markup strings one after another with a blank paragraph line inbetween:

_("<b>Notes:<b> %s") + "\n\n" +
_ngettext("This task <b>blocks %(number_of_parent_tasks)d task.</b>",
    "This task <b>blocks %(number_of_parent_tasks)d tasks.</b>",
    number_of_parents) + "\n" +
_ngettext("This task has <b>%(number_of_children)d sub-task.</b>",
    "This task has <b>%(number_of_children)d sub-tasks.</b>",
    number_of_children)"

...or something like that. With a bunch of if conditions to figure out whether to show the various strings parents string and/or the children string.

From a performance standpoint, I don't know how expensive of an operation querying the number of parents/children is on every task hover, though... there's a risk of spamming I/O or CPU resources because of the way mouse cursors generate a gazillion events while you move them, so maybe this would need to be put behind a timer, like what was done in commit 510a7a4 for live search.