1) Non-U8 alpha color spaces were not connected to the color
   conversion system. Therefore, one couldn't convert to/from
   them any data

2) The code of their methods were blindly copied from U8 color space,
   therefore all the functions (incl. toQColor/fromQColor/difference)
   just read wrong values from pointers (quint8 instead of quint16 or
   bigger).

3) Now all the alpha color spaces are rewritten using a common template
   and the same code reused for all of them, with correct pointer
   conversions using KoColorSpaceMaths.

4) The new color spaces are converted to the conversion system using
   existing Factory::colorConversionLinks() framework. Yes, they are
   created using factories.

Open Questions:
1) In KoColorSpaceRegistry there are some special cases about persistent
   storage of d->alphaCs and 'OwnedByRegistryRegistryDeletes' flags. I
   couldn't find any info about it, so it might be incorrect.