Labels unhiding from 3D window

[MattTheCuber]

Just noticed this issue with rendering 3D labels. When you hide a label from the 3D window, segment a new object, the label gets rendered in the 3D view again.

Steps to reproduce:

1. Open ITK-SNAP
2. Use the snake tool to create a segmentation
3. Go to the "Segmentation Label Editor" and check "Hide label in 3D window" for that label
4. Use the snake tool to create a segmentation with a new label
5. Once you finish, both labels will render in the 3D window. If you check the "Segmentation Label Editor", it still says the label is hidden.

[MattTheCuber]

Another note: when clicking "update", it updates all segmentation in the 3D window even if none are shown (it doesn't display them, just updates their display object I assume). This is annoying if you have a very large segmentations that take minutes to update. I would suggest either default to only updating shown labels or adding a option/button to only update shown labels.