#2858 ttk::treeview heading height calculation bug

[Emiliano]

Setting multiline text headings in the ttk::treeview widget shows a wrong calculation in heading height. This effect appears after the first time a treeview widget is destroyed. Any newly created treeview will have a heading height corresponding to a single line text.

Demo script:

package require Tk

proc buildtree {tv} {
catch {destroy $tv}
ttk::treeview $tv -columns {hello bye} -show headings
$tv column hello -width 120 -stretch 0
$tv column bye -width 120 -stretch 0
$tv heading hello -text "Hello\n$tv"
$tv heading bye -text "Bye\n$tv"
# this ought to work
$tv heading #0 -text "H\nj"
pack $tv
}

buildtree .tv1
buildtree .tv2
after 3000 buildtree .tv1