Provide support to add estimates to the root estimate

[Kyle Treubig]

If the root estimate is selected (i.e., all other estimates have been deleted), make sure the "Add Child" button is enabled.

Additionally, provide a popup context menu in the estimate tree pane to add an estimate to the root. The label should be something like "Create top-level estimate".

[Kyle Treubig]

Resolved by 1b84151