SCXML specify that the <if> tag contains elements separated by tags
<else/> and <elseif/>.
The SCXML Editor used to nest instruction inside the <else/> and
<elseif/> tags.
This patch implement the correct behavior, so each new inserted element
is appended inside the wrapping <if>.  Note that the new elements are
always inserted as last child of <if>.

Task-number: QTCREATORBUG-17674
Change-Id: I327941bbbd8b0cc04b0c26553257ccb2a24c8306
Reviewed-by: Erik Verbruggen <erik.verbruggen@qt.io>