Add station favicons + Now Playing artwork

[Shakura404]

This adds station icons to lists and the menu bar, and provides Now Playing artwork.
Also auto-fills missing homepage/icon metadata for existing stations (saved back to OPML).

Key changes:

• Store homepage/icon for stations, persist in OPML
• Load icons for internet/local stations and status bar menu
• Now Playing artwork uses station icon
• Auto lookup metadata for stations missing icon/homepage

Tested:

• Debug build, search stations, add to My stations, restart app

[SokoloffA]

I'm not really fond of this feature. I feel like we're overloading the interface, especially since in 90% of cases (at least for me) the icons are either missing or meaningless.
The idea behind my program was to have a visually simple and light application that doesn't distract you from your work. There's just an icon in the menu bar, and the program play the music. Nothing flashes, nothing jumps around, and nothing distracts you from your work.
That's why I add features with caution, and in any case, I make them customizable in the settings, and they are disabled by default, i.e., I preserve the previous simple behaviour and appearance of the program.

Just now users are asking for the light interface to be brought back. But with icons the interface looks "heavy". I think the icons are unnecessary.

I don't think icons are necessary:

1. They don't convey any meaning.
2. They violate Apple HIG; Apple doesn't add icons to menus.