Fix issue:   If subcategory is removed and the subcategory is the only subcategory, then the parent should be listed in the drop down of potential new category

The new logic is following:

      # ## Examle category structure:
      #
      # Category A
      # - Subcategory A1
      # Category B
      # Category C
      # - Subcategory C1
      # - Subcategory C2
      #
      # Merge targets for:
      # A  => B, C1, C2
      # A1 => A, B, C1, C2
      # B  => A1, C1, C2
      # C  => A1, B
      # C1 => A1, B, C2
      # C2 => A1, B, C1