Not a single client of this API actually used the event mask feature to
listen for readability AND writability.
Let's simplify the API and have only one hook: on_activation.
InodeWatcherFlags is an enumeration from the Kernel. To avoid using it
outside of Serenity, add a FileWatcherFlags for FileWatcher, much like
we already have FileWatcherEvent::Type.
This is currently being implicitly including by InodeWatcherEvent.h by
way of FileWatcher.h. The former will soon be removed from the latter,
which would otherwise cause a compile error in these files.
This implements FileWatcher using inotify filesystem events. Serenity's
InodeWatcher is remarkably similar to inotify, so this is almost an
identical implementation.
The existing TestLibCoreFileWatcher test is added to Lagom (currently
just for Linux).
This does not implement BlockingFileWatcher as that is currently not
used anywhere but on Serenity.
