Add support for fetching lyrics in background for selected songs

src/actions.h

@@ -137,6 +137,7 @@ enum class Type
 	ToggleBrowserSortMode,
 	ToggleLibraryTagType,
 	ToggleMediaLibrarySortMode,
+	FetchLyricsInBackground,
 	RefetchLyrics,
 	SetSelectedItemsPriority,
 	ToggleOutput,
@@ -1163,6 +1164,18 @@ private:
 	virtual void run() override;
 };
 
+struct FetchLyricsInBackground: BaseAction
+{
+	FetchLyricsInBackground()
+		: BaseAction(Type::FetchLyricsInBackground, "fetch_lyrics_in_background") { }
+
+private:
+	virtual bool canBeRun() override;
+	virtual void run() override;
+
+	HasSongs *m_hs;
+};
+
 struct RefetchLyrics: BaseAction
 {
 	RefetchLyrics(): BaseAction(Type::RefetchLyrics, "refetch_lyrics") { }