Fold folders up into parent if alone in directory

[WizardOhio24]

Closes #192.
Folds folders up into their parent if they are the only item it their directory. For example:
Before:

After:

[extrawurst]

@WizardOhio24 thanks for tackling this! Very appreciated. But on a first glance I do not see any tests. can you cove this new implementation with a few tests?

[WizardOhio24]

Will do

[WizardOhio24]

Tests added, let me know if there's anything else that should be added/changed

[extrawurst]

I think we need more unittests for the selection moving code, there is still some strange bug with collapsing a folded up path:

[WizardOhio24]

I didn't consider the right and left arrow keys for navigation (only up/down), thanks for pointing that out. It should be an easy fix.

[WizardOhio24]

Bug fixed and right/left navigation tests added.
It was left navigation which was going to the previous index, rather than using the new method (available_selections) in up/down navigation.

[extrawurst]

Really cool thanks!

[extrawurst]

@WizardOhio24 damn just found another bug: left used to jump up to the current parent directory. now it seems to only move up one