Fixes: NB#191067 - MButton: Unable to disable the icon.
Fixes: NB#191067 - MButton: Unable to disable the icon.

RevBy: Daniel d'Andrada
Details: Destroy existing icon if no valid icon is defined in model or style.

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