GuiSymbols: Better handling of empty blocks

GuiSymbols: Better handling of empty blocks

* endInsertRow was incorrectly placed before endResetModel, sometimes leading to
  bad access by the view before it was informed that the model was empty. In
  fact begin/endInsertRows were redundant with begin/endResetModel.

* Empty blocks are not supposed to be shown. This happened when moving the
  cursor. A valid block is now selected intead. Due to the design/optimisation
  of the model update, this requires a first update with an empty
  model. Simplify the old code.