Move button mode to the top so icons will show properly

[chookity-pokk]

In issue #38 I ran into the issue where none of my icons were being inserted after going through most of the demo. Though after running (svg-lib-button-mode 1), I was able to rerun those examples and get all the icons showing. I attached a before/after screenshots to the issue.

[rougier]

Thanks. I think you only need to remove display from the list of font lock props:

(setq font-lock-extra-managed-props
             (remove 'display font-lock-extra-managed-props))