QTreeView: add a property for setting vertical column bars

As a user,

I would like to be able to draw a vertical bar to separate the columns,

so that I can more easily see where to click for a particular column.

The issue is that when the background colours are the same, it can be challenging to notice the boundary between columns. We would need something like what we have for QHeaderView. Those vertical separators can improve the user experience.

This feature was requested from us, but it seems that it was also requested in the past here: https://www.qtcentre.org/threads/8684-how-to-show-vertical-lines-between-colums

API details

To implement this in a way that is not overly specific, we need a property that can define for each cell which border line should be drawn, using Qt::Edges.

However, perhaps the cells should only be drawn around certain cells (cells on or below a certain nesting level?) to allow easy grouping into "sub-header" rows. This needs some careful thinking around the API.

Styling

The rendering of the edges needs to be styleable as on macOS the border would need to look very different from i.e. fusion or classic Windows. We need to add a QStyle API for this, perhaps as a sub element.

Once it's styled, it should also be configurable via a Qt style sheet, as overriding a style just to change the thickness or colors of the edges is just as complex as overriding an item delegate.

Qt Quick Control

Whatever feature we add here needs to also be considered for Qt Quick TreeView.