table: allow sorting tasks by state

[oliver-sanders]

User requested feature, the ability to sort tasks by state.

Difficult to achieve in the tree view (without flattening the tree), easy to achieve in the table view.

We might want to consider adding an "optional" column to the table with a text representation of task state (as this isn't typically needed as the task state is contained in the icon) and use the pre-existing task state order variable as the sort function.