Clear unfillable squares warnings on pressing Clear Board

https://bugzilla.gnome.org/show_bug.cgi?id=732291
Clear unfillable squares warnings on Undo/Redo board

https://bugzilla.gnome.org/show_bug.cgi?id=732398
Clear earmarked numbers on pressing 'Clear Board'

https://bugzilla.gnome.org/show_bug.cgi?id=732399