Reviewed by Xan Lopez.

        [GTK] Use GLib/GIO API for FileSystemGtk implementation
        https://bugs.webkit.org/show_bug.cgi?id=51617

        Complete implementation of FileSystem platform code for GLib. This
        converts remaining API points using POSIX calls to platform-independent
        GLib calls and fixes other compilation errors.

        No new tests. This code will be tested when an implementation
        of beginDragWithFiles is complete.

        (WebCore::JSDirectoryEntry::getFile):
        (WebCore::JSDirectoryEntry::getDirectory):
        * platform/FileSystem.h: Added forward declarations for GLib types to
        avoid GLib includes and changed the PlatformFileHandle to be a GIOStream.
        * platform/gtk/FileSystemGtk.cpp:
        (WebCore::openTemporaryFile): Reimplement using only GLib calls.
        (WebCore::openFile): Ditto.
        (WebCore::closeFile): Ditto.
        (WebCore::seekFile): Added implementation.
        (WebCore::writeToFile): Reimplement using only GLib calls.
        (WebCore::readFromFile): Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@74842 268f45cc-cd09-0410-ab3c-d52691b4dbfc