There are some tools along these lines, but most people simply use Frescobaldi as an integrated editing environment for Lilypond. The most GUI-like tools (including Musescore) are based on MusicXML which is intended as a semantic representation of sheet music, usable for more than typesetting; unfortunately, there isn't good interoperability between Lilypond and MusicXML right now, only some hacked-together attempts at basic import and export.

I haven't seriously tried any of them, but have a look a the list at:

http://lilypond.org/easier-editing.html

Maybe Denemo: http://denemo.org/