Simplify right-click track insertion code

[RandomShaper]

The code path used for context-menu track insertion hadn't reset logic implemented. This PR refactors it so it uses the already existing code paths and also avoids some repetition even in the code that remains. The only things preventing this PR from being a true refactor are these:

• This lets reset track logic running (the lack thereof is the issue in AnimationPlayer RESET track is not created (or properties added) if used Right Click Insert Key in timeline.  #55438).
• The cases of TYPE_BLEND_SHAPE and TYPE_VALUE use to include _clear_selection_for_anim() in their undo script. I'm not sure how safe is to stop doing that.

Fixes #55438.

I've smoke-tested this, but this definitely needs more testers.

[akien-mga]

@RandomShaper Did you mean to close this? Seems like you force pushed a no-op branch to your PR branch, which closed it.

[RandomShaper]

No, I didn't. Thanks for letting me know. According to Git, everything was up-to-date regardless I had a diverging local commit. I've rebased and re-pushed and it looks good now.

[akien-mga]

Poke @TokageItLab

[Arnklit]

I just tested this. And it creates the Reset animation and adds the track the first time I add a new animation property and add a key to it, but any subsequent tracks with keys I add don't get added to the Reset animation.

godot.windows.editor.x86_64_Fnvfbqp4sZ.mp4

[RandomShaper]

I guess I need to rebase this. Involved code likely changed a lot since the last push.

[RandomShaper]

I've rebased, but I'm not sure if other code involved has changed in a way that further changes are required, so TIWAGOS.

[Calinou]

Tested locally (rebased on top of master 71699e0), it works as expected. I tested this with various track types (Property, 3D Rotation/Position/Scale, Audio and so on). Automatically added tracks from the inspector still work correctly.

Code looks good to me.

[akien-mga]

Thanks!