Add 'Block mapping rule' type objects, allowing to further customise the Block generator.

- 'Block mapping rule' will be allowd to be added to 'Library class folder' objects.

- It will be possible to define a 'Property name' and "Property Value'.  The process will test the Channel property name defined against the property value defined.  

- If the test matches, the block generation process will take the block defined as a child of the 'Block mapping rule'.

- It will be possible to have several 'Block mapping rule' children of a same 'Library class folder' object.

- When no mapping rule will exist, or if all mapping fail, the process will use any block defined as a child of the library class folder. (So same as current process.)