Add support for #SENOTECHANGE command

[vivaria]

TJAPlayer3 implementation details (https://github.com/IepIweidieng/TJAPlayer3/blob/gh-pages/tja.md#senotechange):

• #SENOTECHANGE can be placed at the start of a measure, or in the middle of the measure
• It will only apply to 1 note immediately after the command
• It will override any automatic Don2/Don3/Ka2 assignment (from note clusters)

How it works in tja2fumen:

• #SENOTECHANGE will be tracked in the TJA measure object
• It will split the measure into sub-measures (similar to gogo/bpm/etc.)
• It will only be applied to the first note, after which it will be overwritten
• We set a manually_set flag to ensure that the manually-chosen value doesn't get overwritten.

Fixes #69.

[vivaria]

sakurada gave the 👌 so i'll merge this and create 0.9.0 :)