floating-bar: ellipsize the filename separately from the details

This patch does three things:
- It adds API to the floating bar to have primary and details labels
  instead of a single label. The primary label is ellipsized separately
  from the details, so it makes a good fit for a filename.
- Modifies the code setting status messages in NautilusView to split
  them into two separate strings
- Removes obsolete code in NautilusView and NautilusWindowSlot that was
  setting long statuses for the old non-floating statusbar

https://bugzilla.gnome.org/show_bug.cgi?id=660695