Merge euclidean and step sequencer

[Chris Lowis]

At the moment the euclidean sequencer is very separate from the step
sequencer, even though it is sequencing the same sounds/notes. This
means I'd have to duplicate functionality to allow, for example, notes
to be toggled and track muting in the Euclidean sequencer.

Perhaps it would be possible to add functionality to change pattern
length, and fill patterns in a euclidean style to the existing step
sequencer and access those functions by pressing, for example

• .

[Chris Lowis]

I removed the StepSequencer and EuclideanSequencer types and replaced them with a more general Sequence type.