[IMP] base: allow to move an element in inherited view

The `position='move'` has been introduced to move an element in an inherited
view.

It's used as
<xpath expr="//@target" position="after">
    <xpath expr="//@node" position="move"/>
</xpath>

or also

<field name="target_field" position="after">
    <field name="my_field" position="move"/>
</field>

The xpath with a position `after`, `before`, `inside` or `replace` can thus now
have another xpath as *direct* child with position `move`.