Misc #204: Add glx_take_screenshot() & others

- Add glx_take_screenshot() for taking a screenshot. With ImageMagick
  the output data could be viewed in this way:

    display -size [SCREEN-WIDTH]x[SCREEN-HEIGHT] -depth 8 -flip
    rgb:[PATH]

  (#204)

- Add D-Bus command `opts_get string:paint_on_overlay_id` to get X
  Composite overlay window ID. (#204)

- Code cleanup.