mouseinteraction: simplification and coding style updates.
mouseinteraction: simplification and coding style updates.

What I've done:

 * There's just one application class to make the juicy parts stand out
   more. Should make it easier to tear apart and reuse.
 * Vectorized most of the math to be consistent with how things are
   usually done in Magnum (so no separate x/y/z components)
 * Fixed the depth averaging (was using pixelSize(), which is *not* the
   amount of pixels in an image)
 * Removed use of the Magnum:: prefix, using Magnum typedefs for scalars
 * Clarified symbol names, converted names from snail_case to camelCase
 * The non-camera-relative unprojection was used nowhere in the code, so
   I turned it into just a comment
 * Converted the mouse controls to resemble Blender more (muscle memory
   is hard to beat, sorry :D)

Squareys/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples