* scripts/lilypond-invoke-editor.scm (dissect-uri): Handle URIs

* scripts/lilypond-invoke-editor.scm (dissect-uri): Handle URIs
with Windows root in file name.

* scm/backend-library.scm (ly:system): Only redirect output (using
system and shell, ugh) if /dev/null is writable.

* Documentation/pictures/GNUmakefile [PLATFORM_WINDOWS]: Build
windows icon.

* scm/lily.scm (is-absolute?): New function.  Fixes absolute files
on Mingw.

* scm/output-ps.scm (grob-cause): Use it.  Fixes PDF point and
click.

* flower/file-path.cc (is_dir, is_file): New function.  Actually
use result of stat when available.

* flower/file-path.cc (find): Use it.  Refactor.  Fixes Mingw,
absolute file name and root.

* scm/lily.scm (PLATFORM): On mingw, slashify getcwd.

* scm/editor.scm (get-editor-command): Typo: do not apply editor.

* flower/file-name.cc (slashify): Bugfix, do substitute if no
slashes in file name.