v1.6.3.0

tracking 1.6.0 to 1.6.3:

scoumbourdis#38: Bug fix: required_fields doesn't work for relation_n_n fields
scoumbourdis#465: Translation for Spanish Uruguay by @mlopezcoria
scoumbourdis#468: Remove PHP 7.3 warnings
scoumbourdis#469: datatables theme - update table fails after delete
scoumbourdis#470: Update Polish translation by @tikky
scoumbourdis#442: Searching in grid with value 0 is not working
scoumbourdis#458: Updated Lithuanian language by @dgvirtual
scoumbourdis#463: Updated Spanish language by @CarlosPinedaT
scoumbourdis#441 Adding clone functionality - contribution from @portapipe