Partial table selection must not be possible

[Reinmar]

Partial table selection must be fixed to a complete table selection.

Note that it's only partially related to the fact that we don't support cell/row/col selection yet. When the selection starts outside of a table, then even in the future we should rather extend the selection to the entire table if a part of it is selected (because mixing a real selection with a fake one will be hard). The behaviour will change only for cases when selection starts inside a table and spans a couple of cells. Right now, we have to prevent that somehow (either shrink the selection to a single cell or expand it to a whole table). In the future, this will trigger row/col selection mode.