Add a note to the ::destroy docs

Clarify that ::destroy is not suitable for saving widget state.

https://bugzilla.gnome.org/show_bug.cgi?id=710793
Editable cells demo: Add new row at cursor

Adding rows to the bottom of the list is confusing as you cannot see
them if the window is small so it is not apparent that anything has
happened. Fix this by adding the new row immediately below the current
row and set the cursor on the new row so it is ready to be edited.

https://bugzilla.gnome.org/show_bug.cgi?id=721939