Less string copies from nautilus-files.h

During the properties window redesign I realized that there are many places where strings get copied, where a normal constant pointer would suffice. I moved these changes to a different MR in order not clutter the other one up.

The commits themselves can probably (partially) be squashed, but I kept them separate for easier review. If there are further candidates to be turned into returning const pointers, I can gladly add them.