Changed build file and modified closable tab listener

1. Changed buildfile.xml of PlanningPoker project to be able to include images via getResource() function
2. Icons for the buttons

Reported-by: lhnguyenduc & bnurbekov
Signed-off by: Long Nguyen <lhnguyenduc@wpi.edu> & Batyrlan Nurbekov <bnurbekov@wpi.edu>