GtkTreeView: change child repositioning to preserve locality

GtkTreeView: change child repositioning to preserve locality

If there was a piece of text in the cell, then when the edit
entry is shown for that cell, it should have a piece of text
in it roughly at the same location.

Therefore, when child widget is enlarged (child preferred
size exceeds cell size), extra width should be added by
extending either left or right edge depending on text direction.

If after that the child sticks outside of the treeivew visible region,
try to push it back inside (breaking its alignment with the
cell), again, giving preference (i.e. adjusting it last)
to either left or right edge depending on text direction.

https://bugzilla.gnome.org/show_bug.cgi?id=765471