grappling_hook_start: Switch to Creep Outro when returning from powerup

[wjt]

The desired behaviour is that when the player returns to this scene
after collecting the powerup, a music transition to Creep Outro is
triggered. Creep Loop will be playing, so this allows it to reach its
natural end, then play an outro, and transition to Still in preparation
for the next scene, void_grappling.

Previously this was achieved by having a second copy of this scene,
grappling_hook_end, which set BackgroundMusic.clip to "Creep Outro".
However since commit b86ea32 this scene
was removed. As a result the music was switched to Creep Intro when
returning to this scene.

Change BackgroundMusic.switch_to_clip() into a setter for
BackgroundMusic.clip, so that it can be used before
BackgroundMusic.play() has been called (from _ready()). (This has the
nice side-benefit that you can now test AudioStreamInteractive
transitions in the editor by hitting Play in the inspector then
adjusting the Clip property!)

In grappling_hook_start, add a ClipSwitcher, triggered by the player
being teleported, in the same way that we disable the opening dialogue
in that case.

Update ink_combat_round_6 to use ClipSwitcher, matching
ink_combat_round_3.

Resolves #2110

[github-actions]

Play this branch at https://play.threadbare.game/branches/endlessm/wjt/grappling-hook-start-switch-to-creep-outro-when-returning-from-powerup/.

(This launches the game from the start, not directly at the change(s) in this pull request.)

[manuq]

I didn't realize that I broke this, sorry.

Is very nice to test the music transitions in the editor. I spent some time clicking Play and then changing the Clip.

The clips I see now in a playthrough are:

• Creep Intro in grappling_hook_start, the first time.
• Creep Loop in the levels in between.
• Creep Outro in grappling_hook_start, when returning.