Simplify (hopefully) code for moving selection in filetreelist

[cruessler]

This is a potential follow-up to #2496.

It changes the following:

• It adds get_new_selection as the single function that iterates over the list’s items to get the next selection based on a range and a step size.
• It adds enum Direction as a more explicit alternative to up: bool.
• It removes some loops in favor of iterators and their APIs.
• It removes some duplication with respect to whether the new index is different than the current one.

The only thing I’m worried about is that the ranges passed as arguments to get_new_selection potentially are too clever or unintuitive. If you think that’s an argument against this PR as a whole, feel free to close it! 😄 I still wanted to open it in order to get your feedback.

I followed the checklist:

• I ran make check without errors
• I tested the overall application

[extrawurst]

looks good to me!