treeview: make sure separator nodes have a height > 0

In case the theme doesn't set a height/min-height for the treeview separator the treeview drawing gets confused and draws rows on top of each other depending on the redraw area.

This is due to gtk_tree_view_get_row_height() assuming that a node with a height <= 0 is not set and not a separator and it will default to the expander size.

Ideally gtk_tree_view_get_row_height() would know if it operates on a separator, but there are too many calls/levels, so just make sure the separator height is at least 1 (Adwaita already sets "min-height: 2px", so no change there)