Add textmime macro

There is quite a number of functions that need to be defined for a
text mime-type to flawlessly interact with the rest of the system.
If the mime type is not included in Base, this is quite tedious, so
add a macro that defines all the necessary functions appropriately.
Merge branch 'kf/textmime' into kf/osx1092
Don't introduce extra mmacosx-version-min for osxunwind