TreeView: Fix indentation, fix size of expand-icon's hitbox, add option to choose spacing of item content

[robkri]

This PR contains some changes to TreeView that seemed usefull to me when I was using the widget.

• All items of the same depth level now have the same indentation. Before, only items with the same parent were aligned and there was an 8-pixel-offset between children of different parents when one of them contained expandable items and the other did not.
• The hitbox for the expand icon of each item now uses the item's full height and is three times wider than the actual icon. Before it was restricted to the icon's size (8px by 8px) which was not very comfortable to use. Now it corresponds to the implementation in the explorer of Windows 10/11.
• Now TreeView offers the option to choose whether the items of a TreeView use more or less spacing. The examples shown in the Microsoft documentation use a wider spacing than the actual implementation uses for the explorer in Windows 10/11. Both options are available now, as shown in the attached image.
• The build method of TreeViewItem now contains some short comments to make it easier to find individual parts of each item like the selection checkbox or the expand icon.

Pre-launch Checklist

• [ x] I have updated CHANGELOG.md with my changes
• [x ] I have run "dart format ." on the project
• [x ] I have added/updated relevant documentation