Merge qualifiers for duplicated properties in UI sort page

[jmformenti]

When we read properties from the UI sort page, merge the qualifiers when a property is duplicated. Currently, we are only considering the qualifiers for the last definition. For example:

* P536 (Date of Artifact) MILESTONE_BD
:: qualifier P787 (* Precision of begin date) MILESTONE_BDQ
* P536 (Date of Artifact) MILESTONE_ED
:: qualifier P788 (* Precision of end date) MILESTONE_EDQ
:: qualifier P721 (* Source) MILESTONE_BASIS

currently is parsed as:

{
  '536': ['P787']
}

we want:

{
  '536': ['P787', 'P788', 'P721']
}

Note: Avoid duplicated qualifiers in the final result.