Fix sound manager not being stepped by GUIEngine

[Desour]

Fixes #13660.

(The dtime calculation is not perfect here, i.e. we still always sleep for a fixed time. I'll leave that to another PR.)

In case you're worried that sounds are streamed too infrequently if the window is unfocused:

• There's [NOSQUASH] Move soundmanager into its own thread #13633 for this.
• We could pause all sounds if the main menu window is unfocused, like in singleplayer game. I'm undecided in this.

To do

This PR is a Ready for Review.

How to test

• Put a music file at devtest/menu/theme.ogg.
  Side note: Putting menu_music.ogg in one's sound pack doesn't work anymore since Allow for Game-Specific Menu Music #11241. :/
• Launch minetest, and listen.

[sfan5]

lgtm