refactor: consistently use time_t t... variables for times in sound system, stats, application and player controls
refactor: consistently use time_t t... variables for times in network system, scheduler, gui

The network used to cast GetTime() to int, but GetTime() is an unsigned long. This might cause problems if GetTime() returns big integers (see #251). To solve this, the StdSchedulerProc interface had to be extended with another function in order to eliminate the magic return value -1 of GetNextTick for "no scheduled execution".
add SoundAt function in  C4Script (part of #238), document

also, extend the documentation for Sound with previously undocumented parameters; threw out undocumented and also unused "multiple" parameter.