MultiLayer Export Improvements

[Simarilius-uk]

The basic functionality is already in the plugin but it needs polish, things identified so far:

• Be able to change the mltemplate of the layers, preferably from a dropdown with the other ones in use prepopulated
• repath the mt if modified?
• Be able to save the mlmask layers with the mlsetup
• Check the colors, things seem to be shifting slightly in round trip (purple straps on panams trousers for instance)

[DoctorPresto]

Should we be exporting an mlsetup and mi instead of .mt? It would be relatively simple to set up definitions for a drop down in the same way I've done for collision materials, there's quite a few more .mt but with proper search/filtering it shouldn't be an issje