Change many FRAME_X... macros to FRAME_WINDOW... or other

Change many FRAME_X... macros to FRAME_WINDOW... or other
non-platform-specific equivalents.
[WINDOWSNT]: include w32term.h, fontset.h and define X specific
functions and macros as their w32 equivalents where
non-platform-specifics are not available.
[HAVE_X_WINDOWS]: Change most of these to HAVE_WINDOW_SYSTEM.
(x_create_gc, x_free_gc) [WINDOWSNT]: Add W32 versions.
(clear_font_table) [WINDOWSNT]: Call w32_unload_font.
(frame_update_line_height): Use macros to access f->output_data.
(defined_color): Remove FIXME comments; fixed.
(x_face_list_fonts, prepare_face_for_display): Put X specifics into
#ifdef blocks. Add WINDOWSNT blocks.
(Fx_list_fonts): Use macros for accessing font data.
(set_lface_from_font_name): Different default fonts for X and WINDOWSNT.
(font_scalable_p) [WINDOWSNT]: Treat wildcard XLFD_AVGWIDTH as
scalable for backward compatibility.
(realize_tty_face) [MSDOS]: do the same for WINDOWSNT.
(syms_of_xfaces) [WINDOWSNT]: Allow scalable fonts by default.