Favorites dead entries

[gammillweg]

 * Nemo version (nemo --version)
 * Is issue with desktop or windowed nemo?
 * Distribution - (Mint 17.2, Arch, Fedora 25, etc...)
 * Graphics hardware *and* driver used
 * 32 or 64 bit

Issue: Deleted items (file) in Favorites displayed.

Steps to reproduce: Under Linux terminal "rm" an item in Favorites. Open Favorites and it is still displayed

Expected behaviour : The item (file) is gone, it should not be displayed in the Favorites list

Other information: Suggested fix: on user click of an item, check to see if it exists, if not, ask if should
be removed from the Favorites list. If yes, remove it. Alternately provide a way to request that an item
in the list be removed from the list. (Currently there isn't any way to clean up the list short of recreating
the file ("touch" the name), which then shows it as a starred favorite, and remove it from favorites via Nemo.)