SortWidget: Fix moving of items over several positions

Previously items weren't actually moved, but swapped with the item
at their new position. That works well if you move an item for one position.
However, if you move an item over several positions, you won't get the
expected behaivor.

frontend/ui/widget/sortwidget.lua

@@ -416,8 +416,9 @@ end
 function SortWidget:moveItem(diff)
     local move_to = self.marked + diff
     if move_to > 0 and move_to <= #self.item_table then
+        table.insert(self.item_table, move_to, table.remove(self.item_table, self.marked))
         self.show_page = math.ceil(move_to/self.items_per_page)
-        self:swapItems(self.marked, move_to)
+        self.marked = move_to
         self:_populateItems()
     end
 end
@@ -475,15 +476,6 @@ function SortWidget:_populateItems()
     end)
 end
 
-function SortWidget:swapItems(pos1, pos2)
-    if pos1 > 0 or pos2 <= #self.item_table then
-        local entry = self.item_table[pos1]
-        self.marked = pos2
-        self.item_table[pos1] = self.item_table[pos2]
-        self.item_table[pos2] = entry
-    end
-end
-
 function SortWidget:onAnyKeyPressed()
     return self:onClose()
 end