MPrisPlugin: Limit string and URL lengths in metadata

Limit strings (artists, etc) to 200 characters, and for URLs disallow anything that isn't http(s) and clamp it to 2038 characters (which is the supposed limit of URLs that Chrome allows).

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@davidedmundson @aacid @fvogt @bf

2038 randomly found on https://saturncloud.io/blog/what-is-the-maximum-length-of-a-url-in-different-browsers/#maximum-url-length-in-different-browsers

We might want to catch those on the extension side already so we don’t end up sending massive amounts of data to the host but let’s make sure the host is always a safegurad.