Sculpt mode Gravity feature from GSOC 2010 by Jason Wilkins.

Reviewers: sergey, brecht, campbellbarton, jwilkins

Differential Revision: http://developer.blender.org/D89
improvements and simplification to gravity code:

* world to object matrix is calculated for view vector as well, reuse
that.
* if no orientation exists use world negative axis instead of object
negative axis for gravity.

there's also the issue of mirroring, it may be wise to invert the
gravity effect there but it may be better to have artist feedback on
that.
Code Cleanup: use static for sky_model_data.h
RNA: Add pixels property type
Fix T37692: Follow active quads failed on unselected-active-face
Radial control operator support for pixel properties
UI: remove text margin when a slider is aligned to another button

Paint sliders had too much space on right (aligned with pressure icons)
Merge branch 'master' of http://git.blender.org/blender