Add IsLoaded checking & NotLoadedException for properties of Spotify types

[ChrisBrandhorst]

In order to prevent AccessViolationExceptions, we need to add IsLoaded checks to properties of Spotify types. If IsLoaded returns false, those properties should throw a NotLoadedException.

The following types should be addressed:

• Track
• Album
• Artist
• Image
• Playlist
• User

[Alxandr]

Also, for performance-reasons (due to locking), IsLoaded itself should not be called, rather the sp_*_is_loaded should.

[Alxandr]

Actually, thinking about it, I think that maybe all API's should throw a NotLoadedException when relevant, simply because that's a LOT easier to track down than a random NullReferenceException that doesn't occur if you set breakpoints (cause then things have time to load).

[ChrisBrandhorst]

Agreed. I should have the NotLoadedException somewhere in the commits, since I already added it once, but removed it later :-)

[Alxandr]

Lets just remake it. It's too messy drawing things back from old commits imho.