BUG: Preserve custom color tables on scene view restore

Moving the color table information into files on disk created a bug (analogous to the same situation with fiducials[1]) that when a scene contains scene views that contain custom color tables the scene view restore wipes out the colors in the color node in the main scene. When a MRML file is read, the files on disk are not reread to fill in scene view nodes, so the scene view color table node is empty. This fix adds a check for if the scene is currently restoring a scene view, and if so, don't proceed with the copy as it will result in an color table with the correct number of entries (as the number of colors is saved in the MRML file) but all black and unamed.

[1] fiducials were fixed in this commit: http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=24376 https://github.com/Slicer/Slicer/commit/1440084bbdd9d11c1e9e1f195087f521a91af186

Issue #3992