"Expand All Tasks" stops at first layer

[picsel2]

Description

Clicking "Expand All Tasks" from the main menu only expands one layer of the collapsed tasks.
Repeating this action yields another layer.

Info

This is a report against the branch from getting-things-gnome#894

[diegogangl]

This is also related to the lazyness of the treemodel. Those subtasks in the collapsed tasks don't "exist" in the model until we expand them, so their own subtasks also don't exist