KateSessionManageDialog: reset Tree position after moving the header section

Even tough the session list isn't actually a tree, after moving the first column in second position, the list thinks it has now the tree expander in the second column. This will interfere with QStyles, if they have a different look for the background of the first,middle and last column (for instance if it has rounded borders) its QStyleOptionViewItem::viewItemPosition will be wrong, as it will have End for the first column (because is followed by a column with the tree expander) and First for the second column (because is the column with the tree expander)

Setting -1 as tree position will reset it to be in the first visual column

The visual but is visible with the Oxygen style and an upcoming patch in Breeze which rounds items backgrounds