WebCore: Remove Nix-specific changes from FileSystemGtk.cpp

This patch is still in the effort of reducing the diff from Nix to trunk.

We add a FileSystemNix.cpp which is basically the one provided by the Gtk
port without the Windows-specific bits and without depending on the
WEBKITGTK_API_VERSION_STRING define.

Reviewed by: Trust me, I am an engineer.
WebCore: Remove Nix-specific changes from RunLoopGtk.cpp

This patch is still in the effort of reducing the diff from Nix to trunk.

We add a RunLoopNix.cpp which is basically the one provided by the Gtk
with the specific Nix bits.

Reviewed by: Trust me, I am an engineer.
WebCore: Remove Nix-specific changes from SharedTimerGtk.cpp

This patch is still in the effort of reducing the diff from Nix to trunk.

We add a SharedTimerNix.cpp which is basically the one provided by the Gtk
with the specific Nix bits.

Reviewed by: Trust me, I am an engineer.