Display more columns in the "Export Project to Dxf" dialog (take 2 )

[DelazJ]

This PR is an alternative approach to #57155: instead of a QgsLayerTreeView it uses a QTreeView for automatically displaying all the columns in the dialog. This seems to be the actual widget type used in the other dialogs that display layer tree along with other columns (see #57155 (comment)).
It reverts #56082 that looks more like a hack to me, as we need to adjust width bounds whenever number of columns changes.

dxfColumnsSize.mp4

The bad news: it doesn't work. I can get the dialog with columns visible and I could export a project. But some interactions with the dialog (e.g. double-click in a cell, resizing dialog not the columns) lead to a freeze then a crash (of my whole computer). An issue with the Q_ASSERT below but this is a too high level for my c++ understanding.

QGIS/src/app/qgsdxfexportdialog.cpp

Lines 151 to 154 in 8d9c68b

	QgsVectorLayer *FieldSelectorDelegate::indexToLayer( const QAbstractItemModel *model, const QModelIndex &index ) const
	{
	const QgsLayerTreeProxyModel *proxy = qobject_cast< const QgsLayerTreeProxyModel *>( model );
	Q_ASSERT( proxy );

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 66d3ed9)

[github-actions]

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