Provide option to try and split bent notes to improve support of complicated bends in GP6

[fholger]

Unfortunately, compared to GP5, the support for bends in GP6 is rather limited. It only supports elementary bends, i.e. normal bends, release bends and bend/release. To author more complicated bends, you have to build them from these elementary types by using several tied-together notes.
This means that we have to split a bended note that can't be expressed with the elementary types into several note values. Since this is delicate and can only be done approximately, this should be optional and only be attempted if that option is selected, or if the output format is gpx.