Have a way to for themes to know all the albums that exists

[shuhaowu]

It would be convenient if the theme can know all the albums that exist, so a navbar can be generated.

Right now, only the current album and subalbums know about this.

If this is fixed, I can likely contribute a theme that I've been working on, as right now it's full of hacks due to the 5 bugs that I've posted.