Follow-up from "Resolve "Improve GUI esthetic"

Description

Our icons are globally all over the place, : different border sizes, different contours, and even different naming styles.

For more information : https://git.ircad.fr/Sight/sight/merge_requests/112#note_65929

Proposal

Write a standard document that states once for all, all of the parameters to be taken into account when creating a new icon.

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)