treeview: make sure separator nodes have a height > 0

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)

Cherry-picked from !614 to master