Add important modules to API doc

[pllim]

~~As discussed in #224, which must be merged first before this can proceed.~~

@ejeschke -- I think the most important modules for developers using Ginga (as far as API is concerned), are...

• [ ] ginga.AstroImage
• [ ] ginga.canvas
• [ ] ginga.canvas.types
• [ ] ginga.Control
• [x] ginga.ImageView (see #237)

Also see #182.

These also need to be documented for cross-references to work in the API above

• [ ] ginga.AutoCuts
• [ ] ginga.cmap
• [ ] ginga.ColorDist
• [ ] ginga.imap
• [ ] ginga.misc.Callback (see #85)
• [ ] ginga.misc.Settings
• [ ] ginga.RGBImage
• [ ] ginga.RGBMap
• [ ] ginga.util.io_rgb