Need public APIs to reload symbols.dat and swkb.dat.

[PCMan]

We need public APIs to reload symbols.dat and swkb.dat.
Otherwise, there needs to be some built-in timestamp checks to reload the files as needed.
Providing APIs to users to reload the files seems to be easier.

[czchen]

The scenario of this API is?

I think the easiest way to do so is to create another ChewingContext and replace the old one.

[PCMan]

Since we allow user-specific symbols.dat and swkb.dat, it's possible that the configuration tools might update these files while libchewing is running.
Creating another ChewingContext works, but this looks more like a hack.

[czchen]

@PCMan Just hack an API chewing_reload to reload everything in https://github.com/czchen/libchewing/tree/feature/chewing_reload, is that what you need?