Fix cursor blinking for TextInput and TextEdit

Before commit fb339b21

 relied on the simple
transparent rectangle node to remain invisible. After that commit we used the
regular rectangle node, which doesn't seem to like toggling the color between
transparent and solid black and therefore the cursor was always visible.

As advised by Gunnar this patch implements a much simpler logic: When the
cursor is supposed to be invisible, we just don't create a scene graph node for
it anymore.

Change-Id: I7b0e173f6d37997559ee0911f37903efdb14847f
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>