[O] suggestion: sparse tree heatmaps

[Ilya Shlyakhter]

Suggestion:

Currently, the sparse tree shows entries in proper context but not in order (e.g. by due date); the agenda shows them in order but out of context.
To get both, show a sparse tree but indicate the relative order of the entries by a heatmap color (e.g. blue to red) that would correspond to the
entry's position in the corresponding agenda.