Store and show recently-used categories list

    * Adds Category data class, "categories" table & content provider
    * When search filter is empty, show last 25 used categories
    * Once we have used categories, hide the skip message.
    
    It's also possible to do a prefix search on the local database of
    recently used categories, but I haven't included it in this commit
    as I'm not satisfied with the resulting UI.