Issues undoing mesh color changes

[stefaniapedrazzi]

As already reported in #11, there are some issues undoing operations with meshes for which the color has been changed.

For example:

1. undoing a color change (yellow -> blue)
   -> the mesh remains blue

2. undoing a delete of a blue part
   -> the mesh appears as yellow, in the part viewer the color is set to yellow, but it seems that internally the blue is stored because changing manually back the color to blue doesn't work the first time. It works if set it to blue, then to yellow (visible color matches internal color?) and then again to blue.