When searching in Assistant, we now dispatch some background jobs to
query the whole filesystem. Activating a result will use the Desktop
launcher's default way of opening that file or directory.
When launching Terminal via Taskbar we change to the users home
directory. For consistency, let's also `chdir` to `/home/anon` when
launching apps via Assistant's AppProvider.
If there are no search results in the list, we shouldn't do anything
when you try to active the selected result, since there isn't one.
Fix this by using an Optional<size_t> to store the selected index.