files-view: Say "Trash is Empty" only in trash:///

Empty trash widget is displaying for empty folder inside trash instead of Empty folder widget.

eel_uri_is_trash only checks whether the URI starts with "trash:", that's why it also returns true for empty folders inside trash.

Create eel_uri_is_trash_root to distinguish trash root (trash:///) from empty folder inside trash (trash:///emptyfolder)

Closes #1367 (closed)