table: Fix sorting of tables using the header
A recent change in Qt enforced that the HorizontalHeaderView's selectionModel always creates a header proxy model if an abstractitemmodel is used for the HorizontalHeaderView's model. This apparently broke setting the current column when using our own header proxy model. However, we actually do not need the current column and selection model handling, as it is just an integer property to keep track of the column. So instead drop the selection model and track sort column manually.