Support opening lyrics in an external editor

[jacquesh]

This is supported by LyricShow3, so it would just be feature parity.

We currently watch the lyrics dir for changes so we can automatically detect those and update the loaded lyrics. However that doesn't work if we're configured to save to a tag, so what we should probably do is create a temp file, open that in the external editor and then when the editor closes, read back the temp file and update lyrics wherever they should be updated.

[H2Swine]

External editor is on my wishlist yes. There are several possible ways to let user invoke it - one is another right-click item, another is an "Open external" button in the editor window. Why not both. (Except the work needed to implement it, of course.)