Add the ability to get any meta-tags by textual name

[Wohlstand]

Regarding stale #36, there is an idea to add an ability to get any meta-tag just by its textual name. The solution at the pull request wasn't effective, but the idea is good.

So, the way on how to implement that:

• Make the cache of all tags while loading the song
• When requesting a tag, seek in the cache, don't request the tag from the library every time, that will waste the computer time a lot
• Extend the existing Mix_MusicMetaTags structure, make it be able to store the key=value cache which can be used to store loaded tags