plugin: track undo state for serialized data

[JoepVanlier]

• Adds an API endpoint (ysfx_fetch_want_undopoint) to poll whether the JSFX asked for a manual undo point for the serialized state to be made (invoked through sliderchange(-1), polling it clears the state).
• Exposes undo/redo for serialized state to the plugin UI. I would have preferred to have this go over the DAW undo system, but most DAWs do not store the serialized state as presented in getStateInformation as part of the undo state.

For a few versions, this system will be disabled by default in the plugin. It can be enabled by setting the configuration flag ysfx_maintain_serialization_undo to 2 (the default value is 1) in %APPDATA%\ysfx_saike_mod\ysfx_saike_mod.prefs. Note that the plugin should not be running when changing this config, as the plugin will overwrite it with the default upon closing.

To do:

• Expose a config flag to turn this system on/off.
• Verify that the serialized state actually changed before storing and throttle the undo state generation considering that there may be pathological JSFX.