Fix layout binding to map instances when field name contains dot(s)

Binding a map instance to a layout is already possible, but it may not work ok when the map key name contains dots as the expression built by the layout system will not add surrounding brackets to the field value in that case (this makes it possible to bind directly sub elements of a given field or element following usual EL expression syntax, and tp map a document context map key by using the field definition contextMap['request/comment'] for instance).

=> Another kind of field value should be used for these cases. The most natural one would be to quote the map key, for instance:

<field>['my.key.value']</field>

But this is not currently resolved ok as it will build the expression #

instead of #

.