[Qt] Tree Widget

[corranwebster]

Implement the Qt Tree widget.

Fairly straightforward other than the complexities of the QAbstractItemModel for a tree. We store references to node objects on the QModelIndex objects to avoid having to walk up and down the tree constantly.

For the widget warning check, Qt doesn't know whether a cell contains a widget or not until it is actually visible, so the warning gets issued on expanding the rows, not on display of the table and we needed to adjust the tests slightly to accomodate this.

Like the Table, we have with evenly-spaced non-changable column widths for now (see #4109).

Includes a fix of #4111 since I needed that for manual testing. It also includes a fix to a couple of fixtures which should have been async.

Ref #3914.
Fixes #4111.

To Do:

• screenshot
• issues with tests which reparent widget.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct