[1946] Enable child extenders in the View DSL implementation

[pcdavid]

This allows downstream projects and applications to provide their own
sub-types of the DSL types (e.g. new WidgetDescriptions) and they will
be correctly detected and handled (provided they are properly
registered).

Bug: #1946
Signed-off-by: Pierre-Charles David pierre-charles.david@obeo.fr

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[sbegaudeau]

I'll wait for the build to merge it