Added confirmation dialog box whenever one closes a Planning Poker new game tab.

Reported-by: lhnguyenduc and bnurbekov
Signed-off by: Long Nguyen Duc <lhnguyenduc@wpi.edu> and Batyr Nurbekov <bnurbekov@wpi.edu>
Merge pull request #72 from hlong290494/sbl-dev

Added confirmation dialog box for new game tab close button.