Rename private method `where` to avoid conflicts

A public `where` method will be added to Yt::Actions::List in the
next commit, so we can use another less important name for a private method.
New .where method to filter account.videos list

For instance, it is now possible to call

    account.videos.where(q: 'query')

to only return the videos owned by account with 'query'
in the title or description.