feat(music): add css selector for button contents

docs/modules/Music.md

@@ -136,6 +136,7 @@ and will be replaced with values from the currently playing track:
 | Selector                            | Description                              |
 |-------------------------------------|------------------------------------------|
 | `#music`                            | Tray widget button                       |
+| `#music #contents`                  | Tray widget button contents box          |
 | `#popup-music`                      | Popup box                                |
 | `#popup-music #album-art`           | Album art image inside popup box         |
 | `#popup-music #title`               | Track title container inside popup box   |

src/modules/music/mod.rs

@@ -154,7 +154,12 @@ impl Module<Button> for MusicModule {
         info: &ModuleInfo,
     ) -> Result<ModuleWidget<Button>> {
         let button = Button::new();
-        let button_contents = gtk::Box::new(Orientation::Horizontal, 5);
+        let button_contents = gtk::Box::builder()
+            .orientation(Orientation::Horizontal)
+            .spacing(5)
+            .name("contents")
+            .build();
+
         button.add(&button_contents);
 
         let icon_play = new_icon_label(&self.icons.play, info.icon_theme, self.icon_size);