Don't try to open another menu window if there already is one.

* Fixes crash when using Gobe Productive toolbar.
registrar: Move more classes to libstorage_kit_mime.a

Move MimeSnifferAddon, MimeSnifferAddonManager, and TextSnifferAddon to
libstorage_kit_mime.a, so we can reuse them.
TextSnifferAddon: Use MIME DB directly

A DatabaseLocation is passed to the constructor and used to verify that
the sniffed MIME type is installed instead of BMimeType::IsInstalled().
This makes the add-on independent of the default MIME DB.
mimeset: Use TextSnifferAddon when using a custom MIME DB

That addresses the problem that packaged text files aren't correctly
identified (ticket #9972). All concerned packages have to be rebuilt,
of course.