Add support for multi fields relations

[volterra79]

Depends on:

• https://github.com/g3w-suite/g3w-admin/pull/596
• https://github.com/g3w-suite/g3w-client/pull/159

Demo project --> project.zip

• father_relation

• child_relation

• father_relation / child_relation

G3W-ADMIN Configuration --> Configure and activate layer editing

QGIS Configuration ---> 9.3.8. Relations Properties

[Raruto]

Tested with: editing-multi-fields.qgs