New app icon, packaged with a menu entry for Linux

The icon is a plate wearing headphones with a red Nordic house cookie
on it. It replaces the default Godot icon, so the game window, Windows
and macOS builds use it.

Linux programs can't carry their own icon, so the build now also makes
MusicPlayer_<version>_linux.tar.gz with the game, the icon as a PNG, a
.desktop menu entry and install.sh, which installs them for the current
user. The bare Linux binary is still published.

RELEASING.md

@@ -11,10 +11,19 @@ same version in the About window.
 The `Build` workflow (`.gitea/workflows/gitea-build.yaml`) then:
 
 1. runs the tests,
-2. exports the Windows and Linux builds,
+2. exports the Windows and Linux builds, and packages Linux as
+   `MusicPlayer_<version>_linux.tar.gz` with the icon, an app menu entry and `install.sh`
+   (Linux programs can't carry their own icon the way a Windows .exe does),
 3. if no tag exists for the version yet: uploads the builds to the package registry
    under `MusicPlayer/<version>/`, and creates the `<version>` tag and Gitea release
    with the builds attached.
 
 Pushes that don't change the version, and pushes to `develop`, only test and export.
 Don't create tags or `1.x.y` branches by hand; the workflow makes the tag.
+
+## Installing on Linux
+
+Unpack `MusicPlayer_<version>_linux.tar.gz` and run `./install.sh` inside the
+`MusicPlayer` folder. It installs the game to `~/.local/bin` and adds it, with its icon,
+to the app menu. The bare `MusicPlayer_<version>.x86_64` is still published for anyone
+who just wants to run the file.