ContentManager uses WeakReference for list

[KonajuGames]

Change ContentManager's internal list of content managers to a list of WeakReference so content managers can be freed by game code releasing all references to them. Fixes issue #848.

[Nezz]

Just one small thing:
++i should be --i

If that's fixed, it's ready to pull. Thanks for the fix sly.

[KonajuGames]

Gah! And that doesn't show up until you add a second ContentManager. Noob error.

[tomspilman]

Possibly this would be a bigger change than we want to do right now, but i'll mention it anyway.

We should consider dumping the list of content managers.... instead have the ContentManager register with a "static event ReloadGraphicsContent" on construction and remove itself on disposal/finalization. Would not require any weak references or manual looping thru content managers.

[KonajuGames]

That would have the same problem that the list had. The event would be
retaining a reference to the ContentManager, thus not allowing the garbage
collector to free it. The event would be the same as the list we have now,
just hidden away inside an event, and we would have less control over it.

[dellis1972]

is this ok to merge?

[Nezz]

Yep

[KonajuGames]

If it works for Nezz's case where he is removing all references to the ContentManager and the garbage collector is freeing the object correctly, then it is all good.