Refactor WindowDrawList into DrawListMut and fix #413.

[nsf]

• Rename WindowDrawList -> DrawListMut. It's not about window draw lists, but
  about background/foreground draw lists as well. The naming was not an easy
  choice, but seems like in rust it's a common convention to add a Mut suffix for
  mutable entities. Imgui-rs already has DrawList and it acts as an immutable
  reference type for rendering implementations to consume. Hence the name
  DrawListMut, which becomes a mutable reference to draw list with methods
  to modify it.
• Add Ui::get_foreground_draw_list(). Same as Ui::get_background_draw_list()
  but for foreground.
• Add draw_list example which shows the use of all three draw lists
  (window, bg, fg).

[thomcc]

This looks great. It will be a bit until I can merge it since I'd like to check the example on different platforms first but overall very nice.

[thomcc]

Sorry for the delay here, I finally have some time to test stuff today.

[thomcc]

This is great, and works great. Thanks a ton. I don't know when this will be in a release but probably relatively soon.

I'll try and remember to make a comment when 0.7 is cut.