Context

With current implementation, back button is related to history of library browsing, it's not related to the hierarchy of directories (where a given directory can have multiple parents!!).

But now that we have a "home button" (see #169 ), if the user use the home button then the next use of the back button won't bring him back to the Mopidy-Local root directory... Confusing.

Analysis

Save path from root to home directory. Restore that path to history after a click to home button.