#1670 Theme creation tool

[Hugues Peccatte]

Linked to PR https://github.com/phpmyadmin/phpmyadmin/pull/1669, we thought about having a complete tool to create/manage a pMA theme, instead of defining some specific needs.

This tool would help to create a full theme:
- colors
- icons / logo
- navigation tree style
- …

We also could imagine to be able to load an existing theme to duplicate / update it.