Wait until all file system is idle before triggering a new run in watch mode

[lukastaegert]

This is tracking something that popped up during #2259. When a file is modified, rollup will currently wait for 100ms and then start a new run no matter if there were other file modifications during these 100ms i.e. because a more complex save operation is underway.

Ideally, rollup should wait until there have not been any modifications of the relevant files for 100ms. It might make sense, though, to add a maximum for how long rollup will wait until the next run is triggered otherwise there might be situations where no run is triggered at all.

[lukastaegert]

Fixed in master