[suggestion] improved watch stop

[MarcoBuess]

@Zeioth I wonder if it would make sense, instead of tracking the last execution over a grace period to rather capture the buffer that is currently served and monitor the last write time. Then go and reset the grace period when the buffer was recently written, as one could assume that you are still working on the document. I haven't looked up what the default grace period is but serving just randomly stopped after ~1h when I was still editing the document.