[widget Audit] toga.Tree

[freakboy3742]

Audit of Tree.

Includes an audit of TreeSource as well, as Tree uses TreeSource

Builds on #2011 because of dependencies on Table (especially on GTK)

• Modifies the API for insert and append on TreeSource to remove the parent argument. This makes the API for insert/append more consistent with list.
• Adds the ability to have a table with no header row.
• Adds the ability to add/remove columns from a Tree
• Renames on_double_click to on_activate to avoid a specific GUI action.
• Defaults to an empty string for missing data. This message was causing more confusion than benefit.

Fixes:

• Fixes Use of GTK.TreeIter Raises DeprecationWarning #1993
• Fixes SimpleTree widget #1767 (Incorporates the work from Make headings optional #1981 by @bruno-rino)

Related:

• Examples do not support new ListSource APIs #2071

Audit checklist

• Core tests ported to pytest
• Core tests at 100% coverage
• Core docstrings complete, and in Sphinx format
• Documentation complete and consistently formatted
• cocoa backend at 100% coverage
• winforms backend at 100% coverage - no implementation
• gtk backend at 100% coverage
• iOS backend at 100% coverage - no implementation
• android backend at 100% coverage - no implementation
• Widget support table updated
• Widget Issue backlog reviewed