topic: allow unicode symbols in names as long as they are alphanumeric

I decided to relax this logic just a little bit to allow unicode "word characters" (i.e. everything that a unicode regex \w matches). This is still limiting cases that core allows (for branches and bookmarks): core only forbids certain byte values (like null byte, see scmutil.checknewlabel function).

This extra check for topic names could be dropped altogether and we could rely solely on checknewlabel(), but I don't know if there isn't some corner case that topics can't handle. Needs more investigation (and tests).