v2-dev: window: add multiple-window support and expose main loop.

- `window`:
  - Cleanup some documentation.
  - Add support for multiple windows (Fixes #38).
  - Expose the main thread (Fixes #39).
- `example`:
  - Add `gfx_multiwindow` example which demonstrates using multiple windows.
- Notes
  - `example/gfx_stencil` is broken because of a typo/bug in the `gl2` renderer (fix coming shortly).
  - Some details not fully ironed out yet (looking at New being unable to run on the main thread itself).