clients: simple-im: handle proper destruction of objects

Add signal handler for SIGINT to enable graceful exit and
release allocated memory. This resolves the memory leak
observed with Valgrind:

  LEAK SUMMARY:
    definitely lost: 2,520 bytes in 2 blocks
    indirectly lost: 16,820 bytes in 11 blocks
      possibly lost: 0 bytes in 0 blocks
    still reachable: 0 bytes in 0 blocks
         suppressed: 0 bytes in 0 blocks

Fixes: wayland/weston#872



Signed-off-by: Chirag Khurana <quic_ckhurana@quicinc.com>