Reviewed by Xan Lopez.

        [GTK] Fork pieces of RenderThemeGtk that will differ for GTK+ 3
        https://bugs.webkit.org/show_bug.cgi?id=51755

        Split out pieces of RenderThemeGtk into RenderThemeGtk2 and RenderThemeGtk3
        if they are going to differ between GTK+ 2.x and GTK+ 3.x. This is the preliminary
        step to landing support for the new GTK+ 3.x theming API.

        No new tests. This should not change functionality.

        * GNUmakefile.am: Add RenderThemeGtk2 and RenderThemeGtk3 to the sources list.
        * platform/gtk/RenderThemeGtk.cpp:
        (WebCore::RenderThemeGtk::adjustTextFieldStyle): Call the setTextInputBorders static method.
        (WebCore::RenderThemeGtk::adjustTextAreaStyle): Ditto.
        (WebCore::RenderThemeGtk::paintMediaButton): Ditto.
        (WebCore::RenderThemeGtk::adjustProgressBarStyle): ditto.
        * platform/gtk/RenderThemeGtk.h: Make setTextInputBorders a method, so that
        it can be exist in RenderThemeGtk{2,3} and be called from RenderThemeGtk.
        * platform/gtk/RenderThemeGtk2.cpp: Added. A fork of the bits of RenderThemeGtk
        that will eventually be platform-dependent.
        * platform/gtk/RenderThemeGtk3.cpp: Added. Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@74817 268f45cc-cd09-0410-ab3c-d52691b4dbfc