Add suggest next sub / ses including remote project to TUI

[JoeZiminski]

Currently for speed reasons, through the TUI suggesting the next subject or session is done with reference to the local project only (not remote). It would be nice to expose this (it is just an argument on get_next_sub / get_next_ses). However, it should be option as when using SSH (and, in some cases mounted drive) it will be slow

[JoeZiminski]

A nice suggestion related to this is to make the example placeholders in the sub / ses input boxes (e.g. sub-001) actually be the suggested next sub / session. When we can be sure that it is indeed the correct next sub / ses based on the entire project (also see #373).