Creates a new line and inserts it at the beginning of the store. The first
parameter parent can be null.
If that's the case, the new entry will have no parent and thus
be a "root" entry.
The second parameter items is an array with a value
for each column. If your treestore has three columns, the array should have
The method returns a GtkTreeIter which you can use
to refer to the entry row; e.g. to use it as a parent for another row.
If you pass no parameters at all, an empty row with no values will be
created, and you have to use other methods like
set() to change the column values.