Improvement: --glx-swap-method & --fade-exclude

- GLX backend: Add --glx-swap-method, to reduce painting region if the
  driver uses exchange or copy buffer swaps. Untested.

- Add --fade-exclude, to disable fading on specific windows based on
  some conditions. Untested.

- Expose GLX backend options through configuration file. Add fetching of
  GLX backend options through D-Bus.

- Use NULL pointer instead of element count to delimit string arrays in
  parse_vsync()/parse_backend()/parse_glx_swap_method().

- Add documentation about "wintypes" section in configuration file.