Added button with editable MIDI feedback values

[johannesfelber]

This version adds a block called LaunchpadButton to General -> Controls.
This button is an extension of the normal button, which allows defining custom low and high values for MIDI feedback in the block settings. Those settings are useful for usage with Novation Launchpads, since button colors are represented by midi feedback values.

[luminosuslight]

Looks good to me. Thank you very much for contributing! I guess this could help many users, although unfortunately I don't have the time at the moment to create a new release containing this change. Maybe I can find someone to maintain the releases.