Fixes: NB#247480 - MSheet can appear empty if the central widget is changed.
Fixes: NB#247480 - MSheet can appear empty if the central widget is changed.

RevBy: TrustMe.

Details: 1) Do not attempt to replace the sheets central widget if it's the same.
2) Disconnect from the previous sheets central widget destroyed signal, before actually destroying it.

nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch