Add tags lyrics fetcher - #482

As I mentioned in #440 it would probably be best to let this be user configurable. Provide some reasonable default list of tags, but allow users to override it in their config. That way when someone comes along and asks why it isn't finding their lyrics, they can be pointed to the config option, rather than having to make a PR to update the hardcoded list.

Good idea, I currently just use the default lyric tags of the tag lib, which should cover all, but you never know...
I will wait with the implementation until I know how I want to implement the lyrics fetcher, comments to that are very welcome.

The spacing of the new song argument is off, but other than that, I can't help review this. I'm no good at C(++), and consequently I've never dug into this code.

As the author of #273 this seems to be doing the first and most important part of what I originally suggested (reading lyrics from file tags), so I appreciate this PR.
I'm not much good at C++ either, so I'll refrain from commenting on the actual implementation

My implementation question is more how should it behave, so you don't have to be a good c++ programmer to comment on that.
thx

My idea was to have it similarly as you implemented it (if I understand the code correctly): offer it as an additional LyricsFetcher that users can choose from.

As you explicitely asked for comments, here are some quick thoughts:

I'm not sure if it makes sense to write the lyrics also to a txt file as for the other fetchers – they are already locally available and reading the tag is probably similarly fast as reading from the txt file.
I imagine that turning this off for just the TagsLyricsFetcher would require more fundamental changes to the whole lyrics fetching machinery. Because disk space is usually not an issue nowadays, this is a rather small issue and I'm happy with writing them, too. There also might be users that want to have the txt files, e.g. because they use the lyrics files also for other applications.

Another small thing that came to my mind: I think it would make sense to have this one first by default. It is likely the fastest of the fetchers and the chance of getting wrong lyrics is nearly zero. But also here: As long as this is a configurable option, it's not a bit thing for me and others might disagree.

As for the second part of my idea, being able to also write lyrics to the tag, this is more complex and there are multiple ways of doing it. Here are just some quick ideas,

• Offer a new key binding in the lyrics editor that manually saves to the tag (probably the easiest option, but requires manual work when doing so)
• Automatically save to the tag when it was empty before and lyrics were found (as configurable option)
• Allow the user to manually "discard" the lyrics from the tag and fetch new ones from the other fetchers (not sure if this is a good idea)
  As this is only loosely combined to fetching the lyrics, that's something probably better left for a future PR.

Hope I could give you some input, thank you already for your work!

Yay this is gonna be fixed ❤️❤️

Superseded by 9c27eec.