Make it easier to process button *press* only

[helgoboss]

Sometimes you want only the button press to reach the target, not the button release. At the moment there are the following approaches:

1. Set "Source Min" to 1 and "Out-of-range behavior" to "Ignore"
   • Works but raises the minimum feedback value to 1 as well.
   • This is mostly undesired because 1 often means "light on" but probably you want the light to be off when the target is off!
   • This forces you to create an extra feedback-only mapping with "Source Min" set to 0.
2. Set both Target Min and Max to the same high value (e.g. 100%)
   • That way the button release will still be processed but won't reach the target because ReaLearn detects that the target is already "on" and doesn't invoke the target anymore.
   • However, if you press the button down, toggle the target in REAPER (or using another button) and then release the button, you see that the release has an effect!
   • So this is a bit unclean.

Maybe it would be good to provide a way in the "For buttons" section to react to "Press" only. Maybe a dropdown "Process" with options "Press and release", "Press only" and "Release only" - then it would be in line with the idea for #129.