LibCore: Added FileWatcher, a binding for the watch_file syscall

This wrapper abstracts the watch_file setup and file handling, and
allows using the watch_file events as part of the event loop via the
Core::Notifier class.

Also renames the existing DirectoryWatcher class to BlockingFileWatcher,
and adds support for the Modified mode in this class.

Userland/Libraries/LibCore/CMakeLists.txt

@@ -5,11 +5,11 @@ set(SOURCES
     ConfigFile.cpp
     Command.cpp
     DateTime.cpp
-    DirectoryWatcher.cpp
     DirIterator.cpp
     ElapsedTimer.cpp
     Event.cpp
     EventLoop.cpp
+    FileWatcher.cpp
     File.cpp
     GetPassword.cpp
     Gzip.cpp