Give each mod its own content manager, which should let it...

• load any of the game's content;
• load XNB files from its own folder for its own use;
• (bonus) load unpacked *.png files from its own folder.

(Split from #173; see also #255.)

Split #273 into a separate ticket.

Two possible APIs:

• Option 1:

  // load XNB from the mod folder
  helper.Content.Load<Texture2D>("foo.xnb", ContentSource.ModFolder);
  
  // load unpacked image from the mod folder
  helper.Content.Load<Texture2D>("foo.png", ContentSource.ModFolder);
  
  // load 'foo' asset from the game's content manager
  helper.Content.Load<Texture2D>("foo", ContentSource.GameContent);

• Option 2:

  // load XNB from the mod folder
  helper.Content.LoadFromModFolder<Texture2D>("foo.xnb");
  
  // load unpacked image from the mod folder
  helper.Content.LoadFromModFolder<Texture2D>("foo.png");
  
  // load 'foo' asset from the game's content manager
  helper.Content.LoadFromGameContent<Texture2D>("foo");

Option 1 is unanimously preferred per discussion on Discord, so I'll go with that.

Done in develop for the upcoming SMAPI 1.11; pending beta feedback.

Released with SMAPI 1.11.