OWTable: Sort Continuous metas as floats; not strings

Continuous variables inside meta attributes are currently sorted by their string represenation and not by float values. This happens since dtype for metas is object and we map all object to strings before sorting. This PR checks whether the sorting variable is ContinuousVariable and casts values to floats.

Orange/widgets/utils/itemmodels.py

@@ -1015,6 +1015,8 @@ def columnSortKeyData(self, column, role):
                 and role == TableModel.ValueRole:
             col_view, _ = self.source.get_column_view(coldesc.var)
             col_data = numpy.asarray(col_view)
+            if coldesc.var.is_continuous:   # continuous from metas have dtype object; cast it to float
+                col_data = col_data.astype(float)
             if self.__sortInd is not None:
                 col_data = col_data[self.__sortInd]
             return col_data