make mimetype generation largely done by cmake

pvlc only features static mimetype listing (i.e. even the runtime
function returns a compile-time static list of mime types as VLC plays
all and everything anyway). up until now this list was manually maintained
in both the desktop file *and* the backend.cpp, this is now replaced by
*one* list in cmake which is consequently put into a c-string array for
backend.cpp (accessed via mime.h as generated by cmake) and regular
list form (; del) for the desktop file.

this also addresses the issues raised in
REVIEW: 106075