- 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).
