Blender Git Loki

Hair ui simplification:
* There were a lot of settings in the particle panels that made no sense for simple hair and only cluttered up the ui.
* Now these settings are hidden by default unless "advanced" hair options are shown.
* Without advanced options the particle velocity controls are replaced by a simple "hair length" value, which actually corresponds to the grown hair length in blender units.
* Some hair effector options that are actually very useful were not shown in ui. These are now found in the "field weights" panel.