Added decorative pixelart contributed by the community

This adds some nice map decoration crafted by @JOoGraphics and @CombineCR:

![Utility Poles](http://content.open-ra.org/images/screenshots/units_330_19_zh.jpg)

![Russian city houses](http://content.open-ra.org/images/screenshots/units_308_53_ot_thumbnail.png)

![Rice field](http://content.open-ra.org/images/screenshots/units_304_53_ty.png)