* flower/string.cc (substitute): Take two strings or two

* flower/string.cc (substitute): Take two strings or two
characters.  Update callers.

* flower/file-name.cc (File_name): Slashify.

* scm/lily.scm: Use gui-main iso lilypond-main on MINGW.

* scripts/GNUmakefile (SEXECUTABLES): Add lilypond-invoke-editor.

* stepmake/aclocal.m4 (STEPMAKE_WINDOWS): New function.

* configure.in (gui_b): Use it.

* lily/lilypond.rc.in: New file.

* lily/GNUmakefile[PLATFROM_WINDOWS]: Compile it into executable.