Allow specifying fade length in beats/bars/seconds

[timoschwarzer]

This PR allows to pick between beats/bars/seconds for AudioStreamInteractive transition lengths.

• Closes Allow fading by a fixed time in AudioStreamInteractive godot-proposals#11151

Todo

• Backwards compatibility
• Update docs

[timoschwarzer]

I changed the behavior of the options visibility and disabled state a bit to hide/disable controls that are not editing anything right now:

Screencast.From.2024-11-13.23-05-12.mp4

[timoschwarzer]

@RedMser Helped a lot, thank you!

I hope my compat handling of the deprecated get_transition_fade_beats method is fine as it is now.

• It will always return the fade length even if the unit is not "beats" anymore
• This should be fine because when upgrading from an earlier version of Godot, everything has a unit of "beats" because nothing else existed before
• When using the deprecated get_transition_fade_beats on a transition that has been set to use another unit than "beats" I print a warning that the return value is likely not what is expected from the (deprecated) function name and what methods to use instead.

[timoschwarzer]

Some thoughts about renaming the method add_transition to add_transition_with_unit:

• If we would want to keep the old name, the new parameter would need to go to the end of the parameter list. That would mean users need to provide values for use_filler_clip and filler_clip only to provide the unit. Also the unit would not be next to the value anymore:

  add_transition(0, 2, 0.0, 0.0, FADE_CROSS, 2.5, false, -1, UNIT_SECONDS)

  vs

  add_transition_with_unit(0, 2, 0.0, 0.0, FADE_CROSS, 2.5, UNIT_SECONDS)

• The method could be renamed back to add_transition in the next major release because in the end the additional _with_unit suffix has no other reason than preserving compatibility.

[RedMser]

Compat stuff looks good to me now, if the CI also does not complain 😉

[timoschwarzer]

@RedMser thanks a lot for the detailed explanation! 🙌 CI should be fine after the latest push 😄

[timoschwarzer]

Applied suggestions and squashed