Add support for pattern fill in FillSymbolizer

[chrismayer]

We should add support for pattern based fills for polygons (based on an image or predefined geoms).

This needs appropriate extensions for our current style parsers (SLD and OpenLayers) to support this and an UI to set the pattern image.

• [x] Fill pattern in SLD: http://docs.geoserver.org/stable/en/user/styling/sld/cookbook/polygons.html#graphic-fill Solved in geostyler/geostyler#372
• [ ] Fill pattern with OL: http://openlayers.org/en/latest/examples/canvas-gradient-pattern.html

[KaiVolland]

Moved this issue as the work has to be done in this repository.