Add volume_db to the wav importer

[RoyBerardo]

This adds a volume_db setting to the wav importer. Works similarly to normalization where the change is baked into the imported data. Produces a warning if the audio clips when importing.

[Calinou]

Thanks for opening a pull request 🙂

Feature pull requests should be associated to a feature proposal to justify the need for implementing the feature in Godot core. Please open a proposal on the Godot proposals repository (and link to this pull request in the proposal's body).

[RoyBerardo]

Ah thank you Calinou! Just created a proposal for this feature. I should mention - the proposal is for all audio file imports, but this PR is specifically for WAVs. I wanted to do this for MP3 and OGG as well, but it's a little more complicated for those.

[RoyBerardo]

Converting this to a draft. While looking into doing import volume adjustments for MP3 and OGG, I realized it may be better to give AudioStreams a volume_db property which is applied at playback, and to have the import for MP3/WAV/OGG affect that. I have opened a proposal for this, and will be working on a PR.