Create file watching subsystem

Currently, the inotify support is pretty proof-of-concept only. It's directly integrated into the assetmanager, which should be abstracted in the future.

I'm thinking of creating a libopenage/watch/ subsystem which provides some sane api for registering a watch on a folder, file, etc with requested events.

When the watched events happen, generic callback functions should be called so we can use the watches for any file watching makes sense.

In the background, the "sane api" calls the plattform-specific kernel api, but openage can use just one uniform interface. This way we can support OS X, BSD, etc.

Start by reading the current inotify integration in the assetmanager and compare it with the proposed libopenage/watch/ subsystem. Define the uniform watch and callback API around the requested events, then verify that the platform-specific kernel layer can be replaced or extended for OS X and BSD without assetmanager-specific coupling.