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.