Edition - implement Value Relation field edit type for forms

* It is now possible to use in Lizmap data from another layer to fill a combobox. The other layer must be published in WFS
* The checkbox edit type has been debuged
Edition - Add filtering capabilities for value relation edit type
Edition - debug the use of EXP_FILTER when not needed for Value relation
Merge pull request #2 from 3liz/master

3 commits