3D viewer: allow transparency in neuron color

Add a transparency slider to the color wheel (see aschampion/colorwheel)
tied to basic opacity support in the WebGL viewer for manual
single-skeleton and group coloring. Closes acardona/CATMAID#592.
3D viewer: select mesh color with colorwheel

Replace the text input for mesh color with the interactive colorwheel.
Correctly parse the default mesh color to be white, as previously
configured. Correctly create the mesh material for transparency and use
the previously configured opacity of 0.2 by default. Support setting
opacity via the color wheel alpha slider.

Alternative solution to acardona/CATMAID#615.