Make toggling/+/- work for "MIDI/OSC: Send message" targets

[helgoboss]

Should be easy by letting these targets report an artifical current value - then "Incremental buttons" and "Toggle buttons" modes should just work.

[helgoboss]

This should also fix non-working "Reverse".

Wrong: The non-working "Reverse" is a regression because of 2.10.0-pre3 #184 preparations that affects discrete targets.

[helgoboss]

Problem with the "artificial current value" approach: It's one value per mapping. That makes it not suitable for "Incremental buttons" scenarios, which needs 2 mappings.

[helgoboss]

For now that's enough. This change adds the following possibilities, which is already a nice improvement:

• "Toggle buttons" mode works as expected.
• "Incremental buttons" mode works, but in one direction only (because 2nd mapping has another current value and therefore can't participate).
• Control with encoders works without having to tick "Make absolute" which should avoid some "Why!!!?" moments. The effect is the same.

I attempted sharing current values within one ReaLearn instance, qualified by target settings. But there are issues:

1. Keeping the target settings as key in a map requires allocation, which is a no-go at least in the audio thread (important for real-time MIDI conversion).
2. It's possible that the user wants to share one current value even if the target pattern is slightly different.

The cleanest thing to do would be to introduce a kind of group index within which the current value is shared. But this can still be done in future.