Conversion functionality for old Harag-plugin BLEND files

[enenra]

This is related to #46 and #12 .

In order to make the switch from 2.79 to 2.8 for modders less painful, we want to add a button that helps convert an opened blend file over to our new structure.

It should do the following:

• remap materials
• import subparts from additional scenes to main scene
• redo collections and move objects to the correct collections from the ones they get placed in after conversion from layers