Add support for stroke direction

[agourlay]

Two parts are required.

Midi Builder

Apply proper effect on duration for notes which belongs to a beats with a stroke effect.

See https://github.com/agourlay/ruxguitar/blob/master/src/audio/midi_builder.rs#L191 with stroke_increment.

Tab canvas

Draw a down or up arrow next the chord.

See https://github.com/agourlay/ruxguitar/blob/master/src/ui/canvas_measure.rs#L462