implement PING/PONG (fixes #5)

PING sent every ~30s.
flobby will disconnect if PONG not recevied.
Additional fixes:
- debug log file truncated on open (flobby start)
- fixed wrong chat log files path help text in dialog
improve map cache file generation

Using timer event to trigger next gen to make sure flobby process incoming messages, this is important for PING/PONG.
Possible to cancel cache file generation.
dont expect PONG after disconnect