cli: refactor cli into smaller functions

[faxe1008]

Refactor the darktable-cli into smaller functions for better readability.

I wanted to do some automation from the cli, where I also wanted to add an option --style-file to pick the style from a file instead of requiring to pre-import it. Anyways while attempting to make that change I found that it's hard to follow what is happening in the cli so I first refactored it a bit. Follow up PR for the other stuff may follow soon-ish.

[wpferguson]

Perhaps you could open a feature request explaining what your vision is for darktable-cli is so that we can put the changes you are requesting in to some kind of context.

However, changing int k; to int i; seems more about personal preference than readability.

[faxe1008]

I thought that separating the refactoring from a feature change is a good idea, functionality wise this PR should not modify anything.

But a short explanation:
I am planning to self host a kind of archive of darktable preset styles that I enjoy using Github Pages. My idea here is to have a repository with the styles checked in and some refrence images. Via GH actions I run each style over the refrence image and create a simple HTML page where you can preview the styles. This is then published to the Github Pages.
For that to work I need to be able to somehow apply a style from file, either import it via CLI or more convenient apply it directly from file.

However, changing int k; to int i; seems more about personal preference than readability.

Yeah that is true, will go over the change set an try to minimize the delta for this kind of stuff :^).