Setting the visibility of X.volumes did not work properly.

The reason was that injects in the constructor overwrote any prototype defined getters/setters and therefor the special case of the visible-setter for X.volumes was fully ignored. The inject functionality now checks if a setter was previously defined on the prototype and then does not replace it.
Also add X.volume.visible getter since the inject method does not inject getters if a setter was defined.
Enable filtering of labelmaps to show only one label.

Example:

var v = new X.volume();
v.file = 'test.nrrd';
v.labelmap.file = 'seg.nrrd';
v.labelmap.colortable.file = 'colors.txt';

v.labelmap.showOnly = 3; // show only label value 3
// or
v.labelmap.showOnly = [1,0,0,1]; // show only the fully red label
Support the X.volume.labelmap.showOnly property in a X.renderer2D.