[Bf-codereview] View3D Dolly (issue4353051)

[ideasman42 at gmail.com]

Reviewers: bf-codereview_blender.org,

Description:
Recently there have been 2 reports about zoom not working right because
at some point you cant zoom in any further.

This is not actually a bug with zoom but a limitation in blender that
there is no way to move the viewpoint forward.

This patch adds view dolly modal operator.
- Supports zoom to mouse position setting.
- Supports dolly hoz/vert setting.
- Moves by a fraction of the view 'dist', so the zoom distance my be
roughly in proportion to the scale of objects in the scene.

Please review this at http://codereview.appspot.com/4353051/

Affected files:
   source/blender/editors/space_view3d/view3d_edit.c
   source/blender/editors/space_view3d/view3d_intern.h
   source/blender/editors/space_view3d/view3d_ops.c