VOXEDIT: improve orthographic editing

[mgerhardy]

the axis aligned editing that auto-extends the region boundaries in ortho view should maybe become an option - also for non ortho mode. Currently it's weird that the behaviour is different from none aligned editing (go to ortho projection view and e.g. select left, right and so on - now place or remove a voxel

Also see issue #500 and #395

• a58bcbe

See BrushContext::fixedOrthoSideView and AABBBrush::extendRegionInOrthoMode()